i've had a 10 gallon planted tank set up for about months now. it currently holds x3 panda corys, x3 harlequin rasboras, and x4 cherry shrimp as a cleanup crew. ph is 7.5, temp is 80°F, never had any issues with ammonia or nitrates/nitrites and the like. corys have always been very active, eating well, foraging, etc. For the past four days, though, one of them - the smallest one - has kinda just been hanging around in the back of the tank behind the plants, not moving much or interacting with the other two. the other day i noticed he was breathing quite heavily as well, but that seems to have stopped, though he's still hiding out in the back and seems uninterested in the food i drop in (sinking shrimp pellets + bloodworms). only onw of them is behaving this way. thoughts?