You may want to post this in the illness section for more responses. But if you "think" there is enough oxygen in the tank then I would ask if you tested your ammonia, nitrites and nitrates and if so what were the results. Also what type of filtration in what size tank and the type of fish and how many would be helpful. In the meantime I would do a large water change, just don't forget to use a tap water conditioner like prime. So test your water (before the water change), post all the other important information and your water results and we can go from there.
yeah try adding an airstone and maybe a bigger filter or up the amount of gallons ur filter is filtering per hour. but ya the air pump w/ an airstone of any kind might help... shut.. maybe 2 airstones or get the bubble wall ones for maximum bubble. anyways try those things and if you cant up the filter rate.... buy a bigger filter. u might just have too many fish using up all the air