I just have one thing to add:
Air pumps don't increase oxygen levels in the tank directly. They provide agitation of the water surface so that diffusion can occur faster. Generally, there is a higher concentration of oxygen in the air than in your fishtank, so that oxygen will dissolve into the water. This generally goes vise versa with
carbon dioxide i.e. CO2 will diffuse out of the water.
If there was a situation where there is a higher concentration of oxygen in the tank than in the air, than the air pump will effectively help the oxygen diffuse of the water.
So if you had a LOT of plants, lived in a low oxygen area (e.g. on top of a mountain) and a very understocked tank, it might be possible for an air pump to decrease the concentration of oxygen in the tank.
It is probably more likely that there would be a high concentration of CO2 in the air, and the air pump is helping INCREASE the levels of CO2 in the water, and this could make the fish gasp.
P.S. I agree that this situation in particular is more likely a water quality issue as
test strips have been proven to be highly inaccurate.