A5H13Y

Hi! I'm new to the forum (although I've read things here for quite some time) and I'm looking for some help.
I have a 55 gallon aquarium (with a fluval filter, I believe a 404) with 1 bala shark , 2 albino corys, and 3 cherry barbs.
I used to have 4 giant tetras, 2 blood-fin tetras and an algae eater but around November/December they all died one by one without any symptoms. One day - swimming around all normal, the next - dead.
Well since then, my shark has been weird. He would hang out in the corner, tilted downward, then sink down there and struggle to swim back up. I've gone through at least 4 treatments for different stuff. And I've changed the water 50% at a time. Each time he would get better, then a week later he gets worse again. Now he's in the worst shape he's ever been in. It started out, he was tilted slightly downward. Then early yesterday morning he was completely vertical (head up) struggling to swim. By mid-day he was on his side at the bottom gasping for oxygen. My dad put some other treatment in the aquarium since I was gone, and by around 4:00 pm he was back to being vertical. Today, he's on his side, gasping for oxygen. So, I put him in a bucket with 50% aquarium water 50% new water (matched temperature).
The strange thing is, he is the only one that's getting sick (so I don't think it's nitrates or ammonia or something like that). Plus, the corys are breeding, so the aquarium condition can't be too terrible.

Any help would be nice. Thanks!

Shawnie

welcome to fishlore!!
im sorry your bala isnt feeling well!
what are your current readings for ammonia/nitrites?
and what test kit are you using?
just because a fish is breeding or acts normal, doesnt mean there isnt an ammonia/nitrite issue each fish tolerates it diffrently...finding out your exact readings, will help us help you more..with the symptoms you have described, it does sound like ammonia....but lets hope not!!