i tihnk its the indian almond leaf some plants can be very toxic to fish take it out and then do a 30% water change. put some tap safe aswell in the water then do water tests your fish should stop dying:D
hope this helped:)
keeping that fish alive will costs u loads of money i think its time to let him go the fish is very ill and its sad to keep him alive he could be in pain if he does die treat ur tank and disinfect it all ur otherfish should be fine im sorry about the one tho ask me if theres any other signs if...