Discus like clean warm water. I use tap water for my discus, even when breeding. I have an RO machine, and I used to use it a lot for breeding until I discovered something. I had a pair lay eggs in my planted 110. They laid the eggs on the leaf of a sword plant. I have had discus lay in group tanks many times, and I usually ignore them, or move the pair to a breeding tank. I wanted to see what the babies of this cross would look like so I clipped the leaf. I was already raising fry artificially (when needed) so I did my usual, and I waited for a hatch. They hatched at a rate of about 95%!!!!! According to what I had read, this was impossible in my water. People say (don’t you love that expression?) hard alkaline water kills sperm and hardens the egg shell so sperm cannot penetrate. I got out my test kits. The water registered a PH of 8.0 and hardness suitable for African cichlids (I don’t remember the exact number-it is written down somewhere).
Since then I have bred and raised discus in tap water (that of course means something different here in Utah than it does in other parts of the country). Some People still say that discus are less sensitive to nitrates if kept in soft water. I don’t know who these people are, or if they know what they are talking about, but my fish do fine.
I recommend a temperature of about 82 f. If you get warmer you just increase the metabolism and the rate of the decay of waste. I have kept discus anywhere from around 78 f (if you are going on a trip this will lower their metabolism so they need less food) to around 88 f (I had a heater that was constantly on, and I left it that way for months - It had a visible effect on the development of the fins, and I am waiting to see if they are able to breed.

Discus food should be high in protein. This is another area where you will get a huge mix of opinions. Most discus keepers agree that discus thrive on protein. A very popular food for discus is beef heart, but I have never had much success with it. It is a pain if you make your own, and expensive if you buy it prepared. I have had fish get a blockage in their intestine (more than once) from it (there seems to be hard parts in it).
Some people use flake food or a small pellet. I like to see discus get a little bulge in their belly, and flake doesn’t ever seem to do that. It is also messy. There are always little pieces. I often use flake with small fish. It is good to use as one of the feedings if your fish eat it. I have had a hard time getting my discus to eat it (maybe I am just not patient enough to get them to switch). I have been adding flake once a day. I have had the best luck with brine shrimp and blood worms. Some people question the nutrition of brine shrimp, but I grew some huge discus on nothing but brine shrimp. Discus will sometimes eat until they are very full. I have never found this to be a problem with brine shrimp, but the outer shells of the blood worms seems to cause a problem if too many are consumed at one time.
The best way to feed discus is in multiple small (how small depends on how many feedings) feedings. A growing discus should be fed three times a day. An adult discus needs to be feed twice a day. If you are feeding flake food you may want to feed smaller amounts, more often.
Discus become aggressive around feeding time, and there is always someone not getting his share. I like to feed with a cone feeder (especially if there is gravel in the tank, or a lot of plants or other decorations) so the food stays in one place for a long period of time. That way the fish can go and eat until full (careful not too full). The trick is to have several cones spread out around the tank. This way the discus that are less aggressive fish can find a place to eat where the dominant fish are not.
Lots of people swear by garlic or bananas. If you want to mess with it, and your fish will eat it, go ahead and feed it in addition to your other foods.