My male betta fish is about 1 and 3/4 years old. He is normally more active, but it's been 2 days now since he's started lying at the bottom of the tank and only coming up for air. He lives in a 5 gallon tank with a cycled filter and a low flow, temperature at 79 Fahrenheit, and a heater. There is no other animal in the tank or any live plant, just fake decorations. I recently did a 100% water change because yesterday his water was cloudy and even a little yellow. Now his water is clean (I used a water conditioner too) and clear, but he's still lying by the bottom. He is not constipated, but I did notice a small bump under his chin (I do not know what it is, maybe a tumor...?). I think he is sick, but I do not know how to help him or what to do. I try to feed him but he won't eat, even when there is food right in front of him. I'm getting worried he's going to starve.