Reverse Osmosis Filter for Aquarium Water

Updated August 12, 2019
Author: Mike - FishLore Admin
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What is a Reverse Osmosis Water Filter System and should I use it for my aquarium?

Reverse Osmosis and Deionization is a process whereby water is purified as the water gets pushed through some sort of membrane. The membrane traps the impurities and they can remove 90% - 99% of the impurities from the water. The type of membrane you use determines the amount of impurities that the Reverse Osmosis unit will remove.

aquarium reverse osmosis filter

Our drinking water often includes minerals, heavy metals (mercury, copper), phosphates, nitrates, pesticides and herbicides (from farming and lawn fertilizers), chlorine and chloramine. These can all be potentially harmful to your fish. However, most municipalities do a decent job of eliminating most of these impurities from our drinking water. Water treatment plants add chlorine and chloramine to the water to kill any harmful bacteria or other "bad stuff".

One side effect from using a Reverse Osmosis unit is that they will remove some of the "good stuff" along with the "bad stuff". Because of this you will need to add the "good stuff" (minerals and other essential elements) back into the water before using it in your aquarium. There are products on the market called RO Conditioners which are made specifically for this process. R/O Right is one product that contains the essential minerals and other elements that your fish need. It should be noted that these are only used for freshwater tanks because saltwater mixes already contain the essential elements you need.

Is a Reverse Osmosis Water Filter Really Necessary?

If you are planning on keeping a saltwater reef tank with various corals and anemones or a particularly challenging freshwater species such as Discus, it may be a good investment. If you live in an area where you can only get well water or your water source is suspect, it may also be a good investment. It really depends on the water coming out of your tap. Some water treatment centers will send out annual reports of the water quality in your area. If they don't, there are places that you can send your water off to be tested, all for a fee of course.

For most freshwater hobbyists these Reverse Osmosis units probably are not all that necessary. Many tropical fish hobbyists have been keeping fish successfully for years without using a Reverse Osmosis unit. If you're a reef tank keeper though you will probably need to invest in one. RO units are usually fairly expensive and you probably don't need one if you have decent water quality. Research your tap water to determine the amount of impurities your water contains and then make an educated decision on whether or not you really need to purchase one of these units.

Reverse Osmosis Water Filter Maintenance

You will need to perform maintenance on your RO unit periodically. How often hinges on how hard your water is and/or how many impurities are included. You can get an RO flush kit that removes some of the lodged deposits in the membrane and potentially prolong the useful life of the membrane. These flush units are usually inexpensive and can some you some money by prolonging the life of the membrane.

What is Deionization?

Deionization is another type of water purification. The deionization unit works by utilizing a process known as "ion exchange". Without getting too technical, this basically means that it removes the impurities and replaces them with pure water. Do you really need to know exactly how it works? No, not really.

Deionization units are usually used in conjunction with RO units to give you 99.9% pure water. The tap water is usually first pushed through the RO unit and then sent to the Deionization unit for further purification.

Periodic Testing

You will need to test the output water from these units periodically to determine if they are still producing pure water. Get water test kits that will test the General Hardness and the Alkalinity of the water. You want both readings to be 0. If the test readings start to trend higher you will know that you soon need to replace the membrane and/or cartridge. The easiest and quickest way to tell when your Reverse Osmosis filter needs to be changed is to use a Total Dissolved Solids (TDS) meter. I like to change my cartridges out when the TDS measures over 10 ppm or so for my reef tank water.

Still need more information? Check out the video below from BRS which explains a 5 stage RO/DI filter. It is worth watching.

Reverse Osmosis Deionization Tips and Comments

From: Guest
I use RO water from vending machines outside the grocery store. Costs 30 cents a gallon and I use 50 percent RO - 50 percent tap water and my fish are healthy and the tank is crystal clear. I have a sand substrate which tends to push my pH up so the RO water was a simple and cheap remedy. Currently the pH is 7.5 and holding. Would the water from grocery store vending machines be suitable for reef aquariums?
Most likely yes it would be ok for a reef tank. You can pick up an inexpensive Total Dissolved Solids (TDS) meter to check it though. Check your tap water too to see how many dissolved solids are in it. You might be surprised at the difference between tap water and RO water. In my case, the tap water is usually around 180 ppm while the RO filtered water is around 1 - 2 ppm. Fairly big difference between the two water sources. You still may want to invest in an RO unit because you'd probably end up saving money in the long run versus buying it from the store. But 30 cents a gallon seems reasonable if it's good water. Test it to be sure.

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