I have had my delta male betta for around 6 months, he is in a 3.2-gallon tank. I have just purchased a 6.8 gallon as a hospital tank for some guppy issues, once the problem is resolved I plan to give the tank to my betta. The hang on back filter had 2 sponges as filter media, however, I moved one of the two sponges into a somewhat new tank in order to deal with ammonia issues that tank is undergoing (hence the hospital tank). He has been doing great since I got him, I feed small amounts of a variety of frozen and pellet food 3 times a day. His water tested 0 ammonia, 0 nitrite, 5 nitrate on the API test kit as of now, but it tested slightly high a few times since I removed the media. Two days ago I noticed a large amount of his fin mass for unknown reasons as well as some curling. He is now acting very lethargic and laying in his hideout and under the filter intake. I believe the problem is definitely not finrot but perhaps biting his tail because it was so big which I heard was a possibility. His fins are completely clamped up, folded, and bitten looking. He comes up to get some air occasionally and will be fairly responsive but is quickly tired out and retreats to his hides. I have however removed the plants and dosed the water with API aquarium salt as well as lowered the water level as of 4-5 hours ago so he could get air easier. I have no idea what is wrong with him I need help.