Kuro's been getting worse lately. Originally, I thought he simply had a case of tail biting, because it looked like clean chunks of his tails were bitten off and he seemed perfectly fine otherwise. Lately he's been keeping towards the bottom of his tank, eating only one pellet of food per day, and staying by the heater. He doesn't seem to like breathing much, and it's worrying me. I'm not sure what it is, it doesn't appear to be fin rot (but it could be). I added two teaspoons of aquarium salt after doing a 75% water change once he started acting ill, but he seems to be even worse now.
Tank size: 10 gallons
Temperature: 77 degrees F
Water additives: Seachem water conditioner, stress zyme, and two teaspoons aquarium salt
Filter: No (it was giving me trouble so I went without it)
Water changes: I perform 50% water changes every four days