Hello; I think some fish keepers focus on the water test kit results and ignore that there are many other things in the water we do not have any tests for. You also say the water tested --- "since the water parameters are also not bad." Not sure what not bad may mean. Anyway you are feeding stuff that if not eaten or removed from the tank will rot and put decay by products into the tank. Even if eaten the foods will become metabolic emissions from the fish itself..
What I am driving at again is you are probably not doing enough WC. 30% seems too little. You may be looking at the need for 50% or more every two days and maybe even more often to just keep up. I went back thru this thread again and noted a resistance from your posts about upping the WC. If you somehow thought you could do some increased WC for a few days untill things improved and then go back to a lighter schedule, that may be wishful thinking. Let me add that I am not there and do not know the whole situation.
Hello; Any such new change can be a suspicion. I do not have any way to know for sure. Have you check the parameters of the source water with a test kit?
Hello; At the very least be sure to remove any uneaten foods as soon as possible. Give the fish a bit of time to eat and then get the food out so it cannot rot.
Hello; I am figuring the fish is still in a 70 gallon tank with about 55 gallons of water? Still seems too much fish in too small amount of water.
Also have you been keeping the mechanical part of the filter clean? Uneaten food bits and other organic detritus( fish poo and such) will rot in the filter the same as in the tank..
Water parameters which I check are only ammonia , nitrite and nitrate. Ammonia keeps increasing to 0.5ppm the next day after wc and I have been doing 30% WC atleast once in 48 hours.
Nitrite keeps showing 0ppm and nitrate is between 20 to 40ppm (dark orangish color comes in the api result).
I have now reduced the water level to around 40 gallon, since the fish was showing sign of gasping for air and wanted it not to feel any pressure.
I am also scooping out all food waste and fish waste from the tank using fishnet and also using a gravel cleaner to remove things.
I cleaned the canister filter almost a month back, which actually crashed the nitrogen cycle, I didnt clean the canister filter post that, may be will clean one of the canister filters now ( a mild rinse).
The ro memberane change is in the filter using which drinking water for home purpose is also done. I checked the ammonia level in the filtered water and it is zero.