If after testing your water you are happy that it fits the water chemistry required to keep Discus Fish you can do a couple of things. You can simply add a treatment like Tetra Aqua Safe which will take out chlorine, chloride and some metals. You can also use a heavy metal filter which will take out all heavy metals and make the water perfect. Or you can mix the two like I do to save on time and money If your water doesn’t match and you have poor water to start with, you will need to filter it through a Reverse Osmosis unit. These can cost a packet, but are worth it if you need them. This filtration process will remove a very high percentage of heavy metals, chlorine, pesticides, silica, nitrates and most of the Total Dissolved Solids, it will also lower the pH of freshwater. The product water, as with distilled water, on its own is unsuitable for Discus. The reason for this is that there are no salts in this water which would buffer any drops or peaks in pH. What you do is add to it a supplement containing electrolytes and salts, or some de-chlorinated tap water. Better still mix RO water with a percentage of water prepared with a HMA filter. The exact percentages to mix really are determined by your geographical location, but as a rule 75% RO to 25% tap is usually about right.
If you wish to breed Discus Aquarium Fish you need a supply of very soft water, in order to keep the water very soft, i.e. less than 50 ppm; you use less tap water in the mix. But a word of warning, soft water is very unstable and must be changed daily to avoid pH crash.
Water changesIt is very important to change the water in your Discus Aquarium Fish tank on a regular basis, in a display tank I recommend around once or twice a week and with the later for breeding tanks and anything up to once a day for growing on tanks. I like to change in between 20-40% of the water in the tank but in the fish farms in the Far East they change 100% of the water once a day in the growing on tanks with great results. The reason for this is that discus, like other fish stay appropriately sized for the volume of water it is contained in. It is for this reason that wild specimens and those subjected to abundant water changes grow to such large sizes. When changing the water clean the bottom of the tank and suck up all of the debris on the tank floor but try and disturb the Discus Fish as least as possible. Once they feel more secure, you can clean aggressively, but do not cause them stress by being noisy about it, just remember that sounds are amplified in water as it is a much better conductor of sound and shock.