I don't know if my fish is sick or just being lazy. Whenever I check the tank, he's usually lying on the gravel. He's not leaning to the side, but instead is sitting upright. He doesn't seem to be gasping for breath, though his gills are working a bit rapidly. To be fair, I haven't paid attention to that before, so I don't know how normal it is, but as mentioned before, he's not gasping. He will swim around if I agitate him, and he looks fine. Specifically, he's hanging out in cozy places, like under the decorations.
The tank is a 5 gallon with a filter. I tested the water last night and everything came out within normal parameters. The water is on the warm side, since we don't have air conditioning.