Hi new here looking for advice. 75 gallon lightly planted with sand substrate, fluval fx6 and a sponge with airstone, ph is a stable 7.9, ammonia 0 ppm, nitrites 0 ppm, nitrates 5ppm. 6 5-6 in discus, 10 pristela retra, 7 scissortail rasbora, 2 bristlenose, 4 corydora. Tanks been running good for a little over a year. About 2 weeks ago I noticed one of the large discus wasn't eating and his gils were flared out a bit (no redness or visible infection) I wanted to be proactive so I brought a video to my lfs and he thought maybe wasting disease (parasitic) and suggested I dose the community tank with expel-p and see how he responded to that and then possible transfer to a hospital tank if he didn't improve. Things took a quick turn for the worse about 6 hours after I dosed the tank I noticed that healthy fish were laying on the sides and gasping. Did a large water change to get the meds out of the water and dosed with aquarium salt and they started to come around a bit. That was Sunday. None of discus have eaten since then except for one. The others all huddled in a corner and breathing really slow. I've been doing 30% water changes daily. They look healthy but they're not acting it. Any advice to get them to eat and ideas on what could've gone wrong would be greatly appreciated.