Selecting the Right Reverse Osmosis Unit

AquaFX RODI Water Filter

My Personal AquaFX RODI Water Filter

Selecting aquarium equipment can sometimes be a stressful task. You don’t know exactly what products you will need or what brands to stay away from. Other times you may not know what features to look for and which ones are total overkill for your setup. This holds especially true for reverse osmosis systems. RO units, as they are abbreviated, come in a huge variety of flavors. Some are complete bare-bones setups, while others have pre-filters, deionization cartridges, and even their own pumps. So how does one go about selecting the number of stages their filter should have, what brands to avoid, or what items are crucial to providing pure water in the long term?

Basic Anatomy

All reverse osmosis units come with one thing, a RO membrane. The membranes come in a couple variations, but that’s beyond the scope of this article. The basic point here is if the filter doesn’t have a RO membrane, it’s not an RO unit. Brilliant deduction, huh? All kidding aside, there are many types of tap water filtration systems available to aquarium hobbyists, and none of them are as effective as a reverse osmosis unit. The membrane does a spectacular job of removing dissolved solids from the water. Chemicals can be added to tap water to make it “safe”, but who really is willing to take that risk? Municipalities are always adding things like chlorine, chloramine, and other nasty compounds to the tap water that most additives aren’t capable of handling. The RO unit is able to handle most of these compounds, given you have the proper filters, providing pure water to your aquarium.

The RO membrane, for obvious reasons, is the most important aspect of the filtration system. Since it can get clogged easily and replacements aren’t cheap, the membrane needs pre-filters to both protect it and extend its useful life. This protection comes in the form of various sediment filters and carbon blocks. They trap larger particles and absorb chemicals, such as chlorine, that can clog or even degrade your membrane. The selection of the appropriate filters can be confusing (see the explanations of each type of pre-filter below), since each one serves a different purpose. But in general, your RO unit should have both sediment and carbon pre-filters. They won’t last as long as the RO membrane, but they are also relatively inexpensive and can really save you some money in the long run.

Sediment Prefilters for RO/DI Unit

Clean, new sediment cartridge on the left, compared with cartridge after 8 months (approx. 1500 gallons) on the right.

Pre-filter Types

Pre-filter selection can be quite tricky. There are multiple different types of filters, with each type having its own subset to choose from. The first filter most RO units possess is a sediment filter. The biggest issue with selecting from these is which size to get. Some sediment filters will remove anything larger 5 microns, while others go all the way down to half of a micron. The smaller the microns, the more sediment the filter will trap. Keep in mind though that this also means it will become clogged more easily. There’s really no right or wrong choice. One size might filter more thoroughly while another might last quite a bit longer.

The next type of filter usually encountered contains carbon. In general, carbon will remove chlorine (harmful to the RO membrane), VOCs (volatile organic compounds), and other water contaminants, making it extremely important to the water purification process. The downside though is municipalities have started adding chloramine to tap water, which is a molecule consisting of both chlorine and ammonia. The standard carbon filter cannot effectively handle this, so some RO unit manufacturers will offer a different type of carbon, called activated catalytic carbon. This type of carbon has a positive charge added to it, making it more effective at removing chemicals and impurities.

Post-Membrane Filtration

After the water has gone through the pre-filters and the membrane, it’s still not 100% pure. It may have a low level of total dissolved solids, which is still enough to promote algae growth in your aquarium. This is where the deionization (or DI) resin comes into play. Most RO units come with either a single or dual-chamber DI filter, which as its name suggests removes ions from the water. These ions are not captured by the pre-filters or the membrane, and must be removed somehow. The DI resin binds ions and minerals, effectively removing them from the water. The resin will not remove uncharged organic molecules, which is fine because those are handled in the pre-filter and RO membrane. After the DI stage, the tap one contaminated tap water should be clean of impurities and ready for some salt.

Warning: Do not drink water that has been deionized. It can make you sick. If you would like to drink purified water, make sure to take the water after the membrane, but before it gets to the DI resin.

AquaFX Displaying Their RO Units

AquaFX Displaying Their RO Units

Useful Accessories

Now that the filters have been discussed, we’ll move on to RO unit accessories. The best accessories for your reverse osmosis filtration system are the TDS meter, which measures total dissolved solids, and a pressure gauge. Both of these will tell you how effectively your system is working. The TDS output will increase as the filters age and become clogged. The system’s pressure will decrease as pre-filters also become coated with junk in the tap water. If you have very low pressure in your system, a pressure boosting pump will also be quite helpful. Not only will it allow your RO unit to better filter contaminates, it will also help you reduce the amount of waste water produced.

Pointless Accessories

Many aquarium keepers are gadget-crazy, requiring their equipment to not only be top of the line, but also have features that aren’t necessary. The average hobbyist doesn’t need to have a reverse osmosis unit capable of pumping out 300 gallons per day (gpd), or one with half a dozen stages (or individual filters). Most can get by on a simple 4-stage unit. While it is nice to have a system capable of solving all of California’s irrigation needs, it’s really not necessary. You will end up paying through the nose to replace all of the filters and membranes, so it’s not really cost effective for the casual hobbyist. Also avoid horizontally placed DI chambers. These are used to save space, but aren’t as effective as a vertically standing DI canister. Because water is traveling from the bottom of a vertically standing filter, it will come into contact with more of the resin thus making it cleaner.

I hope this article makes purchasing an RO unit a little easier. Be sure to avoid units that have odd sized or proprietary filters as they are very expensive to replace and can only be bought from one source. If you can’t find a manufacturer you like, you can build a unit yourself. Separate parts are available all over the web and you can customize it to fit your aquarium’s needs.


About Author

  • Soymilk

    Hey Brandon,

    I love your blog and make a note to read it daily. I wished I had this info when I was researching which ro/di to buy.

    I think you could add a “part two” or perhaps even a “part 3”. An in depth guide on what to look for would be awesome. Then maybe an advanced article about what membranes are better than others, what to look for in good add-ons, and di recharging. I don’t think brands matter as much since almost all the better ones use the same filters and membranes.

    One of the most reoccurring threads on RC is “help me pick a ro”. So maybe even recommend some units at different price ranges. $100 and under, $100-$200, $200+. Or recommendations based on water usage.

    Although I do agree some gadgets seem a be much, but everything has it’s usefulness. I could be looking at it from the other side.

    Automatic backflush every 100 gallons, monitors to let you know when to change out filters, and autoshut off valves all have their usefulness depending on the users needs. I wouldn’t be so fast to condem them without explaining why they might not be useful to everyone.

    Just my .03 cents. Thanks again for the great blog.

    • thanks for the ideas. i'll see what i can do for a follow up. and as for the gadgets, i'm 100% a gadget junky. my ro unit has several unnecessary additions, but they all have a purpose. i was merely approaching it from a beginner's perspective, since all of the gadgets can be overwhelming.

      btw, did you get the gift card we mailed you for being a winner in the recent giveaway?

  • Soymilk

    Yes I did, and thank you again for the giftcard.

    While researching ro/di systems, I read up on how much a difference a ro membrane can affect your di.

    A difference between 96% and 98% membrane rejection rate could mean a huge time difference in between changing DI resin. I was about to get the vertex ro/di with booster pump but then opted to get one with the better membrane since the water is Houston is so bad. I didn’t want to have to keep on changing out my resin every few weeks.

    • i use an aquafx mako with dual chloramine blasters, 100 gpd membrane, and dual di canisters and it works great. i've been rocking it about 18 months and it is still reading 0 ppm on the tds meter. the filters are dirty though, which i will change shortly, but it's done a fantastic job for me.

  • maxwell

    Hey Brandon,

    Thanks for that post, definitively cleared my doubts about a RO/DI system.

    I'm probably going with a AquaFX Octopus, because i do not have a good water pressure in my home.


  • I have not much time, but I’ve got many useful things
    here, love it!

  • Thanks for information, I’ll always keep updated