How long has the tank been set up?
What's the temp?
Ammonia?
Nitrite?
Nitrate?
If you have a bubbler going, and an
HOB filter, then I suspect it's not an
O2 problem unless the tank is too warm. The other thing is a new tank with high ammonia or nitrite. An uncycled tank will kill your fish if you don't keep your ammonia and nitrite levels below 1ppm, preferably .5ppm.