Best Reverse Osmosis Deionization System for Reef Tanks

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Using tap water in a reef tank is a terrible idea. It’s filled with tons of gunk that may not harm us but will damage our corals, invertebrates, and fish if not controlled. Starting with reverse osmosis, deionized water is the best starting point but who wants to lug 20 gallons of water home from the store?

That’s where finding the best reverse osmosis deionization system for your reef tank comes in. Let’s take a look at some of the best and then we’ll get into how to make sure the system you choose is right for you!

reef tank


1. AquaticLife 5-Stage 200 GPD Reverse Osmosis Water Filtration System – Best Overall RO/DI Water Kit for Aquariums

For those looking to capitalize on RO water, this is a fine choice. It comes with 5-filtration stages, putting it in the recommended range for most tap water, and boasts an easy-to-install design that makes it a favorite. The price is just about right for this great little unit as well.

This is a 200-gallon per day unit, making it suitable for at least 250 gallons of tank provided you don’t mind having to wait a bit to finish the first fill. This will also leave over some water for the rest of your home if you care to use RO for things like cooking and cleaning around the house.

There is one major downside, but it won’t always come into play. 5-stage filtration may not be enough in every area, those with particularly hard or dirty water may want to look for a 6 or 7-stage option instead. It’s also a rather large unit, and you’ll need to find a sink adapter for it since one isn’t included.

That said, this is the ideal RO/DI kit for most people’s home aquaria. Just make sure that you have the space for it, and be aware that it may not remove 100% of the dissolved solids in your tap water.


  • 200 gallons per day
  • Moderate cost
  • Easy installation
  • Readily available filter replacements


  • Not the best for extra dirty water
  • No sink adapter included

2. LiquaGen 7-Stage Aquarium RO/DI Water Filtration System – Runner Up

Designed specifically with aquariums in mind, this RO/DI unit does its job very well. It’s just a matter of making sure that it’s big enough for your home aquarium since the 150-gallon per day output is a bit small for large reef tanks. You can comfortably run a tank of about 150 gallons with a unit this size, or push things a bit and run a tank up to 225.

This is a 7-stage unit, and they’ve got a 0 TDS promise attached. The filters are readily available online and won’t cost an arm and a leg, you’ll be looking at about $50 per year for replacements. It’s also a well-built unit overall, with strong connections and a feeling of high build quality when you’re installing it.

My only real reservation is calling it a 7-stage filter. A normal 7-stage would run a general DI resin bed, then one tailored to cations and anions. Instead, this one pushes the water through three sets of mixed DI resin which is effective but doesn’t really turn it into a 7-stage filter except in a technical sense. It’s ratio of discarded to pure water is also 3:1, which is a bit high for my tastes.

If you can get over that minor technical hangup, however, then LiquaGen’s product is top-notch. It costs a little bit more but it has a 0 TDS guarantee and higher build quality than the majority of units out there.


  • 150 gallons per day for medium aquaria
  • 0 TDS guarantee
  • Easy installation
  • Reasonable size for capacity


  • Not technically 7-stages
  • High waste water to output level

3. Waterdrop D6 Reverse Osmosis System – Premium Option

Let’s get this out of the way before we go any further: few people will need this option’s massive 600-gallon per day output for just their tanks. That said, it makes keeping a tank of more than 600 gallons viable with water from the sink which is impressive in its own right. And it does it while remaining tankless.

This is a six-stage filtration option, but it passes through a barrier inside the unit rather than a series of canisters. It also features some monitoring electronics which can make your life easier, especially the fact that it measures TDS in real-time during output. You won’t ever miss a filter change at any rate. It also has a great 1:2 waste to output ratio.

The downsides aren’t too bad. The unit is expensive, but no more so than those with a comparable GPD rating. It also fills a bit slower than your sink normally would, at a rate of just under .5 gallons per minute. So, you can expect to spend some time running the sink during the initial fill.

Bottom line is that this is a premium option that’s overkill for most tank owners. If you want RO water for multiple applications or have a large tank, however, you’ll find that not having to deal with a large tank is a nice bonus feature.


  • Huge 600 GPD rating
  • Small unit
  • Electronics for monitoring TDS levels
  • Excellent waste to output


  • High initial cost
  • Low flow rate despite high GPD

4. iSpring RCC1UP-AK 100GPD – Runner Up Premium Option

While not as advanced as the Waterdrop D6, the RCC1UP-AK is an excellent unit with a smaller capacity. The reason for the high cost is the pre-pressurized tank, which allows you to get up to 2.5 gallons of water without having to wait for water to permeate the membrane. You can also use larger tanks to get more at a time but you can’t beat the 100 GPD flow rate that way.

For small tanks, however, this is a killer unit with enough water leftover for household use. I’d strongly recommend taking a better look if you’re running a tank of 55 gallons or smaller and aren’t too worried about having an RO kit for the whole house. It also has a re-mineralization filter that raises pH and adds limited minerals back in. This can be advantageous for saltwater tanks, but you’ll have to do your own measurements to make sure it’s a good fit for you. Most users report a pH of about 6.7 and 175-185ppm alkalinity after use. That’s ideal for a saltwater tank in most cases, but it should be noted.

The alkaline filter can be a bit of a pain if you’re only looking for 100% pure water, in which case you can skip this unit. It’s also harder to set up than the majority of those on the list, plumbing the tank is a bit of a pain.

If you don’t mind the post-filtration alteration of the water then this is a great pick. The alkaline filter can even be a bonus in the right conditions, but be sure to test your water so you know how much further to alter it before introducing it to the tank.


  • Very high-quality overall
  • 7-stage filtration
  • Fitted for storage tank
  • Filters are easy to find


  • The alkaline filter adds some pH and minerals back unless bypassed
  • More difficult installation than most

5. Ukoke RO75GP 6-Stage RO Water Filtration System – Great Value for the Cost

Sometimes you just want to get the most out of your dollar, and if you’re sitting in that boat then you’ll be pleased with this option. Among the cheaper RO/DI units on this list, it’s also got a 75 GPD which makes it suitable for smaller tanks. The six stages don’t feature a DI resin cartridge out of the box, but you can replace the alkalizing 6th stage with a compatible resin cartridge if you’ve got a lot of metal in your water.

This unit has a tank as well, which means that you’ll be able to get RO water on demand up to a certain point. It has a relatively low fill rate, which means that you may have to spend some time running buckets to do even a water change of medium-sized tanks but one fill of the tank is fine for a 10-gallon aquarium.

Comparing costs, you know that there will be some disadvantages to this unit. The main one is simply the size. It’s way bigger than a 75 GPD unit should be, some reported it even had to be placed in the garage instead of the sink. The second is just that the system is slow to fill.

Still, if you want a dedicated RO unit for your tank without having to open a second mortgage, then you’ll find this unit is quite acceptable. Don’t expect miracles and you’ll be more than pleased with the money spent.


  • Very reasonable cost
  • Has a pressurized tank
  • 6 stages of filtration
  • Easy installation


  • Unit is large compared to comparable RO kits
  • Tank is slow to fill

6. iSpring RO500AK-BN – Best In-Line Option

I’m a big fan of in-line RO kits, but they do get expensive. They’re able to quickly run water through the permeable membrane and other stages of filtration without requiring a tank to fill. The output is often a bit slow, but it’s much easier to manage than having to wait for your storage tank to refill.

This one boasts an impressive 2:1 water to waste water ratio, putting it among the best options out there when it comes to efficiency. It’s also easy to install and work on, and compact enough to fit under virtually any sink.

It is a bit expensive. It also only has a three-stage filtration process, which means it may not be up to par for particularly bad municipal water supplies. Anything over 350ppm TDS will probably require a more advanced filtration system.

While a bit limited when it comes to particularly bad water, the iSpring RO500AK-BN is an excellent in-line option for regular tap water. Its effortless installation and compact size put it ahead of the competition.


  • Very easy to install
  • Simple filter changes
  • Very compact unit
  • In-line so you don’t have to wait for a tank to fill


  • Only 3-stage filtration
  • Rather expensive

7. Geekpure 3 Stage Portable Aquarium Countertop RO Unit-Best Budget Pick

Not everyone has a ton of money laying around for an RO/DI unit, and that’s okay. You can still get a lot of the benefits for under $100, but you need to be aware of the limitations of such a budget system. This one is the best of the lot, a portable and purposefully designed RO unit for aquariums with a 100 GPD rating.

What you get is great for the money. The unit is easily portable and you can place it anywhere while running the lines through it. It’s not great for use other than in aquariums, but in that regard, it shines. 

It’s a simple, 3-stage filter that lacks more advanced capabilities. It actually works quite well for removing most things from water despite the lack of cost. The unit as a whole takes normal-sized RO filters. You don’t even need a sink to run this one, perfect for filling your tank.

On the other hand, the simple filtration system may not do the best job over 400ppm TSD or so. The unit also requires at least 40 psi of water pressure to function, so a booster pump may be necessary if your tap isn’t the strongest. It also has a terrible 2:1 waste to pure water ratio.

It has some serious limitations, but for the cost, it can’t be beaten. If you’re looking for a cheap, simple RO setup for your tanks then check out the Geekpure.


  • Very cheap
  • Simple to use
  • No faucet required
  • 100 GPD rating


  • Only 3-stage filtration
  • May require booster pump

8. AQUATICLIFE Aquatic Life RO Buddie – Runner Up Budget Pick

For just a little bit more than the Geekpure, you can get this 4-stage filtration unit. It’s a good pick for those who are tight on cash but have particularly nasty tap water, the additional DI resin stage will help strip even more particles out of the water than a standard RO setup.

This one is best suited for smaller tanks, due to the low 50 GPD rating. You can still use it to fill a larger tank but the time it takes can be prohibitive. It has multiple connection threads, including one for a garden hose. This latter one is a bonus if you don’t want to take up space under your sink.

The unit has some problems. It’s very slow compared to most units and the wastewater to RO water ratio is terrible. It’s very touchy when it comes to pressure as well, often underperforming it’s rating if there’s no booster pump. You’ll need at least 45 psi and preferably 65+ psi to get the full rating.

That said, any cheap unit is going to have limitations. On the scale of good, great, best… well, it’s good but it’s also cheap and easy to use as long as you’re aware of the problems inherent to the design.


  • 4-stage filtration
  • Multiple connection types
  • Very cheap for a unit this size


  • Needs high water pressure to operate
  • Slow filling and only produces 50 GPD

9. iSpring RCB3P Reverse Osmosis RO Water Filtration System – Great for Large Tanks

Sitting at 300 GPD, the iSpring RCB3P is a fantastic unit for larger tanks. You can easily handle most residential-sized tanks using this unit. Honestly, if you need something bigger just for your aquariums then you need to look into professionally installed and designed setups instead of DIYing a solution with one of the kits on this list.

The unit is actually designed for borderline commercial use and can easily support a small household and a medium-sized fish tank. This is a 5-stage unit, cleaning the water more than well enough for your fish tank. It may not hit 0 ppm TDS on particularly bad municipal water but it’ll usually be close.

The price you’re looking at isn’t the full price. You’ll still need to purchase a separate storage tank, which makes this one of the more expensive filter setups on our list. It also has a terrible instruction manual, so you may need to supplement it with a quick Youtube course in RO kit installation.

Despite the high cost, this unit is highly recommended for those who have a larger tank. It has a high GPD output, great build quality, and a reputation for great customer service.


  • High output
  • Built very well
  • 5-stage filtration
  • Great for home use alongside tank use


  • High cost with no included storage tank
  • Bad instruction manual

10. Home Master TMULTRA-ERP – Best for Dirty Water

Whether it’s a well or just a bad area, sometimes our water is less than ideal. This system from Home Master is the way to go if you have particularly high TDS or you’re using water that hasn’t been treated for bacteria. It’s a 6-stage setup, designed for bad water specifically.

The final stage in the TMULTA-ERP is actually a UV sterilizer, which kills off any microbes that still remain in the water. This extra layer of protection is particularly welcome for those with well water, or who may have microbes still roaming when the water hits the tap.

This is an expensive unit, and it performs to expectations. The real problem is that the RO unit itself is a bit of a disorganized mess. It’s a bit sloppy but it works well and actually makes it even easier to install. It’s also only a 50 GPD unit.

If you have particularly bad municipal water coming into your home, this unit is worth the price to keep your fish healthy. Just be aware that despite all the tech the design of the unit itself is a bit disorganized.


  • 6-stage filtration
  • Easy installation and filter replacement
  • UV light for killing bacteria
  • Very good for very bad water


  • Very expensive for unit size
  • Disorganized design

How Does a Reverse Osmosis System Work?

Reverse osmosis systems use specialized membranes to strip the ions out of water, creating a neutral and pure end product. They’re commonly used for everything from drinking water to showers to even fish tanks.

Tap water tends to be hard, and it often has the presence of many different types of problematic compounds. For reef tanks, this can be a killer and make your tank unsuitable for the life you want to flourish. While not the case with every tap water system, the vast majority aren’t really suitable to host marine life without a high level of modification.

Reverse osmosis systems force water through a permeable membrane. The membrane catches things other than water as they get forced through. These systems are mainly used to remove ions, so the filter itself often has an electromagnetic charge.

The semi-permeable membrane allows water to pass without allowing the solids dissolved in the water to go through the filter. This creates two streams of water. The purified stream is the one that you receive, while the portion of the water that doesn’t go through the membrane will be discarded. Most systems have some sort of further treatment for the wastewater, raising its pH to prevent damage to pipes.

Many RO filters use a double semi-permeable membrane, with one anion (-) based resin and one cation (+) based resin. These will strip out their respective opposites, lowering the total dissolved solids and removing most of the electric charge of the water.

Most systems will also have more of the usual types of filtration as well. Activated carbon and some sort of mechanical filter for sediment are both parts of the majority of reverse osmosis systems.

The simple answer is that RO systems use a semi-permeable membrane and controlled water pressure to create “pure” water from the standard tap stuff. 

Should You Invest in an RO System?

Yes, if you’re planning on keeping a marine system that’s larger than 5 gallons or so then an RO system will lead to huge savings. For very small systems it’s sometimes easier to just buy the water.

And that’s the rub of it: RO water costs between $1 and $1.50 per gallon depending on the location. It doesn’t take a very large tank to start seeing savings from investing in an RO system for your reef tank.

Water is also heavy and bulky. Buying more than a few gallons at a time can be problematic, depending on your transportation, and will require you to manage to logistics of getting the water into the car, out of the car, and then into the tank. 

It may not seem so bad the first time, but trust me when I say it gets old fast.

Still, you may be better off just buying water if you’re running a single system of 10 gallons or less. RO systems can be expensive upfront and require long-term maintenance and they can be a pain to get hooked up if you’re not a plumber.

Of course, reverse osmosis water can also be used for far more than just your tanks, and you should consider other uses while you’re looking at systems to make sure that you end up with one that’s the appropriate size.

In most cases an RO system just makes sense to own for those who keep any sort of marine aquarium. It saves money and time, making your pure source of base water for the tank available right at home.

How to Pick Your RO System

You should spend some time thinking about what you’re planning to do with your RO system. For smaller tanks, it’s sometimes possible to just get a small filter directly over your kitchen sink but in most cases, you’ll need to get something a little bit more extensive.

Ask yourself the following before you start trying to pick the right system:

  • How large is my aquarium?
  • How much water will I go through weekly with my current water change schedule?
  • Do I have other uses for RO water in my home?
  • How much water will those other uses take?
  • How much water pressure is available in my home?
  • What kind of contaminants are in my local tap water?

It may be easiest to get a system for all of your home water needs, but it depends a lot on your own individual situation.

Filtration Stages

The bare minimum for most RO systems runs three filters. These would be:

  1. Sediment Filter
  2. Activated Carbon Filter
  3. Reverse Osmosis Membrane

This usually gets the water to a reasonable level of total dissolved solids (TDS).

Four-stage filtration is also common. The fourth stage consists of a mixed resin consisting of combined polymers with a positive and negative charge. These are known as DI (de-ionizing) resin. These are added past the standard reverse osmosis membrane and help to strip any remaining ions out of the water.

As a general rule, the differences between the higher numbers of filters is as follows:

  • 5-Stage Filtration- Consists of an additional activated carbon block before the RO membrane.
  • 6-Stage Filtration- Consists of a second DI resin block in line with the rest of the system.
  • 7-Stage Filtration- Replaces one of the DI blocks from a 6-stage system with two DI resin blocks. These blocks will be negatively and positively charged before the water goes through another mixed DI resin filter.

What you need depends a lot on the local contaminants in your tap water. In general, I recommend using at least a 5-stage RO system for a reef tank. The extra activated carbon is great for things that make their way into the municipal water supply.

For particularly dirty water, a 6 or 7-stage system may be required. You can find reports on your tap water through the Consumer Confidence Reports for your region. Most water suppliers put online copies of their reports on the website. If yours doesn’t, then you need to call your water company and ask for a report.

If in doubt, go with the best filtration you can afford. 6-stage and 7-stage systems should be able to strip the dissolved matter out of anything considered safe enough to run to your home.

Sizing and Flow Rate

The size of your system’s tank is a big consideration when you’re trying to pick the right system.

If you’re just dealing with the tank, it can make things easier. In that case, you’ll just need to make sure you can do a water change with the flow rate from one day. You should be able to do a 25% change from just one fill of the tank. Preferably with a decent flow rate so you don’t have to wait all day for the bucket to fill.

When you add in residential water use things get more complicated. Estimates are 50-100 gallons per person per day. That may seem like a big number, but it includes everything you use water for in the home including the toilet over the course of a single day. You can estimate about 50 gallons per person for the minimum required volume.

Flow rates generally increase by tank size in a straightforward fashion. For in-line systems, you’ll want a flow rate that’s at least equal to 25% of tank volume per hour rather than worrying about the tank. This will let you get your buckets filled in a reasonable amount of time but higher is better.

Residential needs are often concentrated so a good guideline is to ensure you can get twice your home’s required water in a 2-hour period. In doing so you can prevent unnecessary waiting time while everyone is getting ready in the morning or evening as the case may be.

Ease of Install

Reverse osmosis units tend to be easy to install for those with a bit of plumbing experience. Smaller units, such as the in-line under-the-sink models, are also quite easy to plumb even for an amateur.

You can find plenty of DIY videos if you want to be a bit more confident in how you handle the task. You may even be able to find one specifically for the unit you’ve purchased depending on the brand and size.

Larger units will be harder to install than smaller ones for the average person.

If you’re not handy at all then you may need to hire someone to put together a full-home unit for you. While it’s not a major task, not everyone is comfortable working that far back on the water lines coming into their home.

The key here is just to know how involved the process is going to be before you get the unit to your home.

Part and Filter Availability

For ease of use into the future, you want to know whether or not parts are available to replace components of the system. You’ll also want to make sure that the filters are either generic (and thus replaceable) or come from a brand that will still be around in five or ten years when you need to work on the unit.

The availability of parts is no small matter. 

If something proprietary breaks and you can’t replace it? Then you’re stuck with a non-operable system.

Likewise, the availability of filters is also important. It does you no good if you can’t find specialized filters that will fit in the unit when the time comes to replace them.

The majority of RO systems are repairable. It’s just a matter of how easy it will be for the homeowner. If you have basic experience with pumps and filters you should be able to replace anything that goes wrong with generic parts in the future, but without a grounding in plumbing basics you may end up being out of luck.

Warranty and Customer Service

An RO/DI unit is going to cost a good chunk of change, no matter what the size and brand. Even the cheapest units will represent a considerable investment for the average person.

So you’ll want to make sure that you’re protected. Ideally from everything, but at the very least you want to be able to replace any parts that come from the manufacturer with defects. You’ll want to take a look at the warranty before you finalize any purchase.

A limited warranty for a few years is better than nothing, more comprehensive warranties are always better in this case. There are a lot of unexpected things that can happen when you’re working with a water system, and you’ll want to make sure that you can minimize the impact of any problems that happen along.

A full read of the warranty should be done before you purchase. You should also do a bit of research on the company’s service if you can. The best warranty in the world, on paper, isn’t worth much if you can’t get ahold of anyone willing to enforce it.

It’s a small consideration for most of us, but I recommend taking it into account. It can be a tie-breaker for someone who isn’t sure which system they want to introduce into their home.

Clean Water In, Healthier Tank

Using RO/DI water for your tank is a no brainer, but it’s always a good idea to see what you can do at home. Getting your hands on the best reverse osmosis deionization system for your home and tank may be a bit of a chore but it’ll be well worth it in saved time and effort.

If you haven’t quite come to a decision yet why not take a look at the AquaticLife 5-Stage 200 GPD Reverse Osmosis Water Filtration System to see if it’s right for you?