Hi everyone, I've got an albino cory catfish that I've had for nearly 6 months who's all the sudden started breathing from the surface, I'v had two other ones in the past who started doing that until they turned almost translucent and died. I test my water weekly, it's a 20 gallon tall tank with 6 cory catfish, 5 male guppies, 2 female platies, a horned nerite snail, and an amano shrimp, this tank probably sounds a little overstocked but my test results from this morning were ammonia 0, nitrite 0 and nitrate 10-15 and I have lots of biological media and live plants. Like with the other two cories this one is the only one who appears to be affected. Also I have an air pump and my filters give plenty of surface agitation. Is there something else about my water that could be affecting the cory, I know my ph is a little high, could that affect anything? I really want to know what's causing this , so I can stop it from happening.