It's a 2.5 gallon tank, I'm not sure about the ammonia/nitrite levels. I don't think they're high because I just got him on Tuesday and I just changed the water on Wednesday. The water looks very clean, I did lower it though so he doesn't have to struggle to come up for air. I don't have a filter in there, but I do have one, it came with the tank I purchased. His gills and everything look ok to me. I'm pretty sure I over-fed him, on the back of the pellet bottle it says to give 3-4 pellets twice a day, which is way too much from what I'm learning now. For 3 days straight, I was giving him 8 pellets a day :( I didn't feed him today, he's very lethargic today, not swimming much and hanging out on the bottom. How long do you think it'll take for him to feel better?? He clearly doesn't look like he's feeling well. I feel so bad for him because he was fine when I got him on Tuesday. I can't wait till he feels better again.