Can you give us a little more info? Tank size, water parameters, water temp, how many discus, what size, other tankmates, what are you feeding, are they eating well..etc etc.
What type of plecos share the tank? Not all plecos are compatible with discus. Excellent water parameters must be maintained. Water changes are a must. Seek help on Discus forums such as simply discus
They are not compatible. Could be a number of other diseases but you will find much more info on that website i gave you. Even something such as background color could brighten them up, but if they are dying stress or disease could be a cause.
Doing the conversions for the US people (because we suck at the metric system)...that tank is about 12g? It sounds like you need a much larger tank, and to get rid of the plecos, first off. You won't be able to keep your water parameters in check with that small of water volume. Somewhere around 50g tank for a group of discus. The temperature is good. You'll want to get a liquid test kit...and test the water...water parameters should be 0 ammonia, 0 nitrites, and probably like 5 nitrates (if you could plant the tank, that would help a lot with nitrates). They also like soft, acidic water...a lower ph. They have pretty specific needs, and having a larger tank will help you help them. I'm not sure you can do much for them right now without a larger tank, sorry to say...try daily water changes until you figure out what you can do.
Here's a good site for more info: http://www.simplydiscus.com/library/discus_husbandry/discus_basics/discus_4beginner.shtml http://www.discuspro.com/care3.htm