Trained or breathing?

[martiniduck]

I noticed that two of mollies were gulping at the surface, when I walked by the tank. My other fish looked normal and were not at the surface. I know am pushing maximum capacity for my tank. I have a 29 gallon with 3 mollies, 3 platties, 3 guppies, 4 corries, and a swordtail. I did hot stock the tank like this. The mollies and platties keep breeding. I have controlled the tank by giving away as much fry as possible. My NH4, NO2, and NO3 readings are all normal. I have lots of water movement at the surface. Two powerheads and my power filter. I am thinking it is more a learned behavior. They are getting into position to eat. Have others noticed this in their fish? Or am I so over stocked that they lack oxygen?

 

[Skyrmir]

Watch the tank from across the room. If the fish get tired of floating there and wander off, they're just begging for food. If they stay there for a while, they're gasping for air.

Even better, throw an air stone in the tank for about 15 minutes and see if they stop.