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.