Reverse Osmosis for Discus

by Rick Owen

While Reverse Osmosis (RO) water is not required to successfully keep discus, for many it is an absolute necessity to breed them.

To read our article on how to breed discus, Click Here.


Many people are intimidated by the thought of setting one up, but they are really quite easy, and quite inexpensive. A good unit that produces up to 100 gallons a day (under ideal conditions) will suffice for one or two breeding tanks. You will need this and a storage container to store and pre-heat the water it makes. When shopping for an RO unit be aware that you will only need a three stage unit (sediment filter-carbon block-and ro membrane). The 4th stage found commonly in many reef systems is for DI resin and is used to remove even trace amounts of minerals not removed by the membrane. This stage is not necessary for breeding discus as we will actually be adding a small amount of minerals back into the product water. Be sure to get a unit with an automatic shut-off float, so that when once your storage container is full, the unit stops making water.

Typical 3 stage Ro unit with Sediment/Carbon Block/RO Membrane

The way in which you install your Ro unit will depend on where you intend to draw your water from. The most common, and easiest, is to get a garden hose bib adapter. This will screw on to a garden hose spigot and reduce to ¼ inch tubing commonly used in most RO units. Another is a saddle valve that clamps onto a water pipe and when tightened pierces the pipe and draws it water from the inserted tap. Either works equally well.

Saddle valve spigot adapter

Once you have the supply to your RO unit hooked up, you will notice that there is two lines coming out of the Unit. One of these lines is for product water, while the other is for waste water. The waste water line should have a flow restrictor attached to it, and some will have the restrictor and a small valve combined. This small valve is an auto-flush valve-more on that later. You will want to run the waste water line down a drain, or better yet, to some sort of container to catch and the waste water for use for things such as watering your garden, or a water source for your non-breeding tanks. Be warned, the flow from your waste water line will be 3x that of the flow coming out of your product water line. Yup, the worse thing about RO units is they waste an incredible amount of water. As long as you find a use for the waste water, this becomes more tolerable.

The product water line you will need to run into a poly drum (food grade and clean) or a brute style trash container. You will also need to add a small float valve to the inside of the container (supplied with the RO units that come with an automatic shutoff). Typically to install this float valve you just drill a ¼ to 3/8” hole and then screw the float valve into the hole and tighten with the supplied nylon nut. You will then connect the ¼ tubing that come from the RO unit (the product water line) into the float valve and you’re good to go.

Typical RO unit with 50 Gallon drum and float valve

How it works

In many aspects, an RO membrane is not a lot different than a sediment filter. The primary difference is that while your sediment filter might filter down the 5, 1 or even ½ micron, the RO membrane filters down to the molecular level. They are comprised of a wound thin polyamide membrane that only lets molecules the size of water (or smaller) through and prevent larger molecules (ie mineral compounds found in water) from passing. The problem with filtering down to such a small level is that without a means of continually flushing the membrane, the membrane would become clogged or destroyed within a matter of minutes. This is why we have the waste water line, and the restrictor valve. As water enters the membrane, it has two options, to pass through the membrane and out the product water line, or out the waste water line. Do to the resistance of the membrane itself, all the water would flow out the waste water line if not for the restrictor valve. What the restrictor valve does, as its name implies, is to insure that the membrane is kept under pressure, yet at the same time allows water to escape. It operates much in the same way as placing your thumb over a garden hose would. This allows purified water to move through the membrane, while allowing an escape route for water carrying the larger mineral heavy water.

Filmtec Ro Membrane

Maintenance and trouble shooting

After you have fired up your RO unit by turning the water supply on, you should let the system run for a few hours and discard the water made. This is to insure any foreign materials or substances that might have been introduced in the manufacturing process are flushed from the system. A common problem with some households is low water pressure which greatly will reduce the water produced. You should try to maintain as close to possible a water pressure of 70-80 psi to insure the best efficiency of the unit. It’s not that the RO unit won’t work at lower pressures, but your product water to waste water ratio will decrease.

Periodically you will want to “flush” the membrane to prolong it’s life. This should be done at weekly intervals and will greatly prolong it’s longevity. This is easily accomplished by units with a automatic flush valve, you simply turn the valve open (parallel to the waste water line) and wait 30 seconds, then close the valve again. This will help loosen mineral deposits that have built up across the membrane and flush them out the waste water line.

Flow restrictor with auto flush valve

Once very three months or so, depending on usage and input water supply quality, you will want to change the sediment cartridge. The carbon block cartridge tends to last a bit longer, but I would change these once every 6 months. The life expectancy of the membrane itself will depend on several factors, from the amount of contaminates that is in your source water, to the amount of pressure to your system, to how frequently you flush the membrane. Period checks with your TDS meter will tell you how well your membrane is functioning. As it ages, the amount of mineral content will slowly creep up in your product water. When it gets to a level that is no longer suitable for breeding, it is time to change the membrane. Be sure to refer to the instruction manual for specific information on how to service the RO unit you have chosen.

The TDS Meter

A TDS meter is nothing more than an electro conductivity meter that measures the conductivity of the water. Trace minerals like calcium and magnesium will affect the TDS (softness). They can be had for as little as $15.00 or you can spend upwards of $150.00 for some of the nicer multi function units. Myself, I just use the cheaper ones, but I make sure I also have on hand a bottle of calibration solution to insure the consistency of the readings I take. They are a very simple instrument to use, typically powered by a single cell 1.5 volt button battery. You just dip the tip of the meter into the water, press a button, and in less than a second you have your reading in parts per million (ppm). Be aware, when using some of the cheaper versions, the temperature of the water will effect the reading somewhat, so be sure to always sample the water at or near tank temperature. Some of the nicer ones will automatically calibrate for this variance.

Another option to the TDS meter is the GH/Kh test kit. Quite honestly, I don’t use one of these, but there are a few that still do. These are much like the API test kits in that they come with two bottles of solution and a test tube. I find them quite cumbersome and time consuming, but they will work. I am told that a good rule of thumb starting point is to aim for a Kh of 4 and a GH of 4.

A good starting point in my opinion to start off with in your breeding tank, is a water with a TDS of 80 ppm. If your lucky, maybe your tap water will be close to this, but many of us are not that lucky. If you find that while attempting to breed with your tap water, that you consistently end up with eggs that go from translucent to opaque, chances are your water is too hard. This leaves you with one of a couple options, you can either go by RO water at a local LFS, some distilled water and the grocery store, or, the only real option, get your self an Reverse Osmosis system (RO).

HM Digital TDS EZ

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  1. August 10, 2015    

    Can i use a TDS meter to measure pH in water, i don’t see you mentioned this function in this article.

    • Rick Owen Rick Owen
      August 13, 2015    

      Hi Maurice,

      While no, a standard stand alone TDS meter will not measure pH, there are multi function meters that will measure both, along with temperature.

      These multi function meters are really nothing more than three different meters packaged within one housing… hth


    • August 28, 2017    

      No. A TDS meter will let you know the reading only. The reading is based on the bad sectors and bacterias, etc in the water. I found this beautiful guide about TDS level. You may have a look –

  2. November 20, 2015    

    Can you tell me about whole home water filtering systems? We have a lot of chlorine in our water and I’m not sure why I should use water pitchers, under the sink filters and shower filters when maybe one filter system will work instead of two or three. I can’t find any independent reviews for whole home water filtering systems on the web.

  3. December 29, 2015    

    Is it possible to use the waste water? Perhaps, to water the plants or wash the dog. How dirty is it?

    • Pat Husband Pat Husband
      March 14, 2016    

      Your plants will love the fish tank water Jerry.

  4. March 17, 2016    

    I usually reuse the used water and filter it to use as vehicle-cleaning liquid.

  5. October 21, 2017    

    Definitely Whole House Water Filtering system is the best alternative on the market . I purchased this unit after doing a lot of research on other companies AND their filters. I believe that Home Master (Perfect Water Technologies) offers the highest quality water filters, made in the USA. Many other brands offer filters and do not disclose where they are manufactured. Many other brands have cheaper filters that don’t last as long. Many other brands don’t get consistent reviews for taste and water quality the way these filters do. The first time you pick and feel the weight of the KDF85 filter, you’ll know that you’re getting what you paid for.

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