IS Reverse osmosis needed for a discus fish tank

The only way to be completely sure of whats in your water is to use Reverse Osmosis or RO water in your aquarium.  This is somewhat high end and some may say not necessary.  However with an increasing amount of unwanted substances in your tap water this is a good thing to look at if you really want your fish to feel at home.  You can control exactly the type of water you have from hardness to pH.  You cannot use RO water as is.  Some use half RO and half tap water, so the pollutants in the tap water are essentially reduced by half.  Or you can use RO that is remineralized, so the trace elements required to keep your fish happy are added back in.  As you can control these additions you can adjust your water to suit your fish safe in the knowledge that there are no unwanted chemicals in your water.  

If your going to do this get a TDS Meter.  A good reading for Discus is around 130, lower if you want them to breed.  It measures Total Dissolved Solids in ppm so its a good guide as to the volume of things other than water in your water. The problem being you wont know what those other things are without further testing.  The water coming out of a good RO filter with a deionising module should be 0 ppm TDS and have a pH of 7.0.

Most units are specified by GPD or Gallons Per Day.  The production of RO water depends on many factors, the main ones being the pressure of your mains water and the temperature.  So dont be disappointed when your first attempt at RO water produces drips or a slow dribble.  No good for a water change unless you can store the water.  So always go for the biggest unit you can as production will be slow.  If your mains water pressure is below 3 bar get a unit with a built in booster pump.  This will up the pressure, speed up production and make it more efficient.  Pure water comes out of one connection, waste water containing all the bad stuff leaves by another.  Its usually about 1 unit of RO to 3 units of waste water.  Pump boosted ROs are more efficient, produce faster and waste less.

To set up you need to connect it to the mains water, direct is best if you are brave enough to cut a pipe. Some come with connectors that can do this.  You will need somewhere for the waste water to go to.  You can collect it for your garden rather than waste it.  You'll need power if you go for a pump boosted unit. 

Its then just a case of deciding what water properties you want and working out what to add back to the RO to get to that point.  You can start adding tap water back in or go down the remineralizing route to make the perfect water for Discus.  Theres obviously no Chlorine in RO so no need to add a dechlorinating solution.  One good way to get an idea of how good your water is, is to check the TDS (Total Dissolved Solids),  From a good RO nit it should be below 10.  If your planning to breed discus TDS starts to get important in combination with the pH, KH and GH.  For a healthy tank of discus do not go just by TDS.  Low TDS implies good quality water, after all your RO water which is all but pure has a very lowTDS, but you cant keep fish in straight RO water. So TDS isnt a single measure of good water.  You still need to get the hardness and therefore the pH correct.

Be very careful with reverse osmosis water.  A Discus aquarium needs constant water properties rather than ideal ones that change frequently.  But they do like low hardness, a constant pH between 6 and 7 and 0 ammonia, 0 nitrites and low nitrates. 

TOP TIP - The lower your kH reading the more susceptible to pH changes your tank will be.  So be careful to maintain a steady kH reading in your tank.  Staying around 5 - 8 will help keep a constant pH.