Ok, it sounds like your fish are either suffering from lack of oxygen or it's just"something they do"
I have a 50 gallon tropical tank which is set up for plants. The advice above about plants getting eaten is not good. I have many species of plants but absolutely no aeration. I have 25 cardinal tetras, 10 harliquins, 10 rainbows, angelfish, breeding pair of rams plus lots more and the tank is thriving.
Maybe a few more details would help.....what do you mean by the "air filter", do you have other filteration, how many fish have you got in what size tank, do you have any plants etc etc
Give us the absolute full details of how the tank is set up and it may be easier to help