This one's got me really stumped.
Lately I've been out of town several days a week so I've been having my brother come over and feed the fish while I'm gone. I have a young Jack Dempsey that's roughly 5.5" long alone in a 29g grow-out tank. This guy is an awesome fish. Every time I walk in the room, he swims right up the glass and begs like a little puppy dog. He's normally a very healthy eater and is quite active.
About a week ago, I got back home, fed him as usual, and was checking my email before calling it a night. Out of nowhere I hear all of this noise coming from his tank and look over to see him having a fit. He was thrashing all around in the corner under the outflow from the HOB filter, crashing into the glass, splashing up out of the water, and crashing into the sand. He calmed down after a few seconds. I assumed that he swam too close to the filter flow and got spooked or something, so I didn't worry about it. He hid behind a big piece of driftwood, but I expected that after being spooked. I turned his light out and went to bed. Ten minutes later, I heard thumping and woke up to see him swimming around like crazy crashing into things again. I didn't know what to do, so I just turned out the light and went to bed.
The next day, he was still hiding by the driftwood acting lethargic. This went on all day, so I did a water change (he was due for one anyway). Next day, he was still lethargic. Actually, he got worse. His fins started to clamp and he seemed to be having trouble breathing. I did another (larger) water change.
All of his water parameters are fine. Temp stays at about 78, no pH swings (steady 7.2), no ammonia or nitrite, nitrate never got above 10 ppm during all of this. The only thing I could think of is that somehow some sort of poison got in his tank while I was away. My brother says this didn't happen, and can't think of anything that could have spilled into the tank. Large water changes and adding a big bag of fresh activated carbon to the filter haven't seemed to do anything.
So, what to do? He hasn't been eating the past few days. Every once in a while, he'll start cruising the tank, and I try to feed him when he does this. Otherwise, he spends all of his time sitting on the sand next to the driftwood, fins clamped. He has done the freaking out thing several times now, and he'll do it if I make sudden movements (this didn't used to bother him at all).
Does this sound like some kind of poisoning? If so, how do I treat it? Is this damage permanent?