Water could be said to be the most important thing in Discus keeping. The water will either make or break your fish. There are some water qualities you will need to maintain for other fish, and then there are those of the Discus. Since the Discus come from the Amazon, during the flood season, the waters rise, and leaves and such from the riverbanks give a slight tea coloration to the water. This is known as Blackwater. You could out your fish in Blackwater if you wish, it brings out the colour in the fish. Personally, I would only use Blackwater with wild fish, such as Heckels.
-- pH --
Discus need a low pH. pH is the measure of acidity of water. A pH reading of 7 is neutral, which could be fine for some Discus. Others, need lower pH's which go into 6. Use a Tap Water Purifier to get the water lower, a mix of tap water and this water does nicely.
-- Ammonia --
Ammonia is caused by rotting of fish waste, food, and even dead fish. You should keep these as low as possible. If you have over .5 ppm, you should take care of it immediately (Ammo - Chips and/or Ammo - Lock 2). Your fish will love you for it.
-- Nitrates --
Nitrates are a toxic material caused by bacteria in the biological filter as it breaks down Ammonia. Once again, as less as possible. Water changes can get rid of this.
-- General Hardness (GH) --
GH is the measure of Calcium and Magnesium ions that are dissolved in the water. This will go away if you use a Tap Water Purifier, of RO unit. Low GH's are recommended for Discus, they enjoy the "soft" water. A good amount is 30 - 100 ppm GH
-- Carbonate Hardness (KH) --
The measure of Carbonate and Bicarbonate ion concentration dissolved in the water. This helps stabilize the pH in the aquarium. Also, if this is stable, so is your pH. A low KH is also recommended, but not with a CO2 pump on for your plants, it will rise quickly. Water changes and soft water will help this lower.
-- Water Changes-- Water Changes are very important. I do an about 15% change every day. If you make it part of your daily routine, it will become easier to do. I know it is not easy for all people to set aside a section of time every day but you will reap the benefits of a healthy tank and fish if you can do so. If you don't have the time, try and do a 25% water change twice every week as the minimum. As a regular routine, every few days you should scrub down the sides of the tank with a clean rag as discus enjoy to leave a slime like material which will build up readily.
-- Filtration -- It is very important to have excellent water conditions. Water filtration could make or break you experience with Discus. Discus water must be filtered quite regularly and over filtering is usually not a problem as long as the turbulence in the water is not to high. If you are staring new with a fresh aquarium, it is often wise to have the nitrifying bacteria colony in your filter already established. You could use a variety of fish for this but people tend to use neon tetras as they are readily available and a good indicator of water quality.