You say your water parameters are okay. What are your water parameters? Discus are hardy fish unlike what you hear if you pay attention to certain water parameters. What is your TDS before and after a water change? Are you using RO water? Tap water will sustain bacterial growth that will not survive in RO water. Clamped fins and breathing hard means the fish may have gill flukes or eroded gill plates from ammonia burn. One thing for certain breathing heavy means they are not getting enough oxygen. Take the fish that have clamped gills out and give them a salt bath immediately, if it continues medicate. You mention the tank is planted. Are you injecting CO2? If you are, I hope you have a ph controller or are aging your water. Fresh tap or RO water is super saturated with CO2, that is why folks mistakenly think their plants are pearling after a water change. If you are injecting CO2 and do a water change with super saturated water, you are drastically changing the ph and may have too much CO2 in the water, hence fish are breathing hard. No matter the cause, first thing to do is get more O2 in the water, place airstones in the tank immediately and then investigate the reason this happened. Do not raise the tank temperature, if anything lower it until the fish are breathing normally. Take the temp down to 78-80 to lower their metabolism and get more O2 in the water. The warmer the temp, the less O2 in the water; the bigger the fish, the more O2 it needs. Very dark discus most usually means sick fish. Good luck.....DC
