Ok, I have not kept discus yet, but I have done tons of research because I plan on keeping them. I'll try to give the best crash course paragraph I can with the knowledge I have on them.
Discus are a schooling fish and require at least a group of 6 to live comfortably (aside from confirmed breeding pairs, which can be comfortable with just the two of them). They require about ten gallons of tank space PER DISCUS. This means that a group of 6 should be kept in a 55gal or larger tank, preferably a 75gal.
Discus require higher temperatures than a lot of other tropical fish. A minimum temperature should be 82 degrees, but can easily be kept up around 86-88 degrees.
Water changes can vary depending on the size and number of discus being kept. Younger discus, under 4", need more water changes because they are still in the critical growing stage of their life. Older discus, over 4-5", can be kept with fewer water changes, but still require pristine water conditions. Younger discus should have water changes that amount to 50% every other day to 90% everyday. Older discus can be ok with water changes that range from 50% once a week to 50% every other day. Either way, the water parameters should never have any ammonia or nitrite (none at all), and should be 10ppm nitrates at the most (preferably never over 5ppm).
Wild discus do require a lower ph, usually in the 5.5 to 6.5 range. However, most discus in the trade now are from domestic or foreign breeders that have been breeding and keeping them for generations in higher ph water. Because of this, these tank bred discus are able to withstand much higher ph's. They do need stable ph numbers though. So, if your water gasses off a lot of co2 or has a ph shift more than .2 (mine rises from 7.6 to 8.2-8.4) over a 24-48 hour period after coming out of the faucet, you need to age your water. 55 gallon plastic food grade barrels are usually super cheap on Craigslist. The main thing is it has to be steady. This is another reason for frequent water changes is because if you let the ph in your tank drop too much, because of the normal tank cycle, and then add new water, even if it's aged, it will cause too high of a ph swing and can stress or even kill the fish.
Feeding discus also is an important part of keeping them. Flakes won't do. When they're growing, and even as adults, they require a high protein diet. This is why many discus keepers feed beef heart and black worms. When younger, three smaller feedings at least three times a day are required. As they get older and larger, feeding can gradually be dropped to once a day.
Tank maintenance is probably where most people slack the most. First off, having an easy to clean filter is very important. Because of this, most discus keepers use sponge filters and/or HOB filters. These should be cleaned of detritus every water change. Because of this, canisters are not recommended and under gravel filters are an absolute no no. The tank needs to be bare bottom for juvies and should be bare bottom for adults. The tank should be vacuumed out after each feeding to remove any solid waste or uneaten food. However, I think that if you keep up with a more frequent water change schedule and arrange your feedings so that you vacuum the tank shortly after your messiest food, such as beef heart, then you should be fine with once a day. Few to no decor items should be in the tank because they can gather detritus that you need to remove from the tank.
OP, if you're still here, I strongly recommend you reed through my post and see where you need to improve your setup. You have a lot of problems and, I hate to say it, but your remaining discus will probably not live much longer and if they do, the damage has already been done to them such that they will never reach their full potential.
I also recommend you check out SimplyDiscus and the following video. This video is well worth the hour and twenty minutes if you're interested in keeping discus.
Best of luck!
