a couple nights ago, I noticed one of my goldfish gulping at the surface for air for long periods of time. At first, I assumed my water parameters were off but I tested my water and everything is normal. The o2 in the water should be good with an airstone and HOB filter. Now its just this one goldfish that does this. But here's the kicker, he only seems to do this at night when the lights are off and I cannot find a reason why online. Anyone know what he's doing?