I've had my zebra danio for 5 months now. Up till now, they've all been fine. In the past two weeks however, one of them has been swimming slowly at the top, almost like he's really really tired. He doesnt get excited like he used to during feeding time, and I almost never see him at the bottom. I'm not sure if its swim bladder disease; I never see him on his side or upside down. He just sits and swims slowly in a corner at the top.
I dont have a kit to measure levels (I'll get one soon!), but I usually do a biweekly 40% water change. Recently I've been doing it weekly. The tank is understocked anyways, so I dont think ammonia levels would be the problem.
What could it be?