I know that Discus are very delicate fish. What size tank do they need? What about their water parameters? Can they be acclimated to soft water or would they end up dying?
Discus like soft water with lower ph than many fish, but perhaps the most important thing is matching your water to the breeders. Discus are not that hard to keep, good tank keeping is the trick. A deep tank is generally accepted as what discus prefer, also I have found that they look much happier in tanks with natural looking decoration than the bare tanks your see tham in so often.
discus like the biggest tank you can get. Lotso of room and they grow up alot faster too. A bare minimun of 40 is what was recomended to me. They like soft water with a ph between 4 and 6, you can go lower but its harder to keep(7 is aceptable but no higher). They can live with the hardness of city water but they'll be much healthier in the softest water you can get(they won't breed in hard water) you can buy water treatment specifically for discus tanks it will simulate their natural water environment, and stain the water a tea kinda colour( I keep tanen root in my tank) THey like a heavilly planted aquarium too but some plants are hard to keep, I have amazon swords in mine they're doing well. You should have a strong filter system aswell. THey like super clean water, its recomended that you do a larger weekly water change than you would with your other tanks. And they like their water very warm
this an excellent websight put together by pros lots of good info on it discus info
IMO a Ph of 4 seems very low since if I remember rightly from my days at school vinegar is Ph3. I've kept discus at Ph7.6, it very much depends on what they are acclimatised to if you take a discus from a Ph7+ tank and put it in a Ph6- tank it won't do it any good.
yeah I forgot about the originating tank conditions..david is partially right about the ph levels. YOu should find out about the water quality where you buy the fish from and try to make yours at least close. If the ph is higher than 7(neutral) then its alkaline try to match your water conditions to the store and very slowly bring the ph down to a happy level for your fish.