Last night while on my computer I heard gravel being "hit" and I looked over to my 20 gallon and saw my rainbow shark, 4-5ins going berserk swimming really fast and slamming his body up against the gravel. The shark was acting like he had something inside him or trying to run away for something. After like 10 seconds he sprang up into the middle and started to drift down not moving as if he was dying. I panicked and I netted him out and put him in my 10 gallon which has no fish in it. The 20 gallon tank is having somewhat of a collapse but the rainbow shark is hardy and has been my longest living fish which doesn't make any sense. The ammonia levels are about .2ppm and the ph is at 7.0. I have 7 barbs in there and they seem unaffected. In the 10 gallon though he looks like he's recovering so my question is, what could have caused this almost near death of him: is it the water conditions, (I do 25% water chabge every other day due to the high ammonia which doesn't want to go down) or is some kind of disease/parasite?