Lethargic Betta; Lack of Appetite

Question

Meris   A couple of days ago my roommate and I went to Petco and bought a couple of male Halfmoon Bettas. They each live in a separate 0.33 gallon tank on our desks, and we change 3/4 of the treated water in each tank every other day. We feed them about four pellets each morning, and remove any uneaten food that evening so it won't cloud their water. Basically, they have the exact same living conditions. My roommate's fish, Ares, is very active. He likes to swim all around his tank, and he attacks his pellets within a few minutes after she gives them to him. My fish Meris, however, spends most of his time lying in a corner of his tank, sometimes on his side. When he does move, he usually pops up and darts frantically around the tank like he's chasing something, then settles back down in his corner. I think he's only eaten three pellets in the two days I've had him, and the pellet I dropped in to tempt him early this morning is still floating in his tank untouched. Meris has gotten more and more quiet during his time here, while Ares seems to be adjusting nicely, becoming lively and curious (he even built a bubble nest yesterday). Meris doesn't have any white spots on him, he's still the same vibrant blue-green color as when I first got him, and his fins aren't fraying; what is wrong with Meris? :(

Answer
Trisha,
I see a couple of problems. Firstly, .33 gallons is not enough water for any kind of fish. The absolute minimum tank size for a male Betta is 2.5 gallons, where a 5 gallon is best. Another thing I see, is no gravel. He should have something at the bottom of his tank. One reason is that is where good bacteria lives that helps keep him healthy. The main problem is no heater. These are tropical fish and need their water to be 80 degrees at all times. In that small of a tank, there is no way to get a heater and for it to keep the water warm. Without heaters, they will become lethargic and will stop eating, swimming, will become stressed, and the get sick. When feeding a Betta it is important to know that their stomach is the size of their eye. 4 pellets might be too many. Also, remove any uneaten food after 2 minutes. Waiting too long will start to cause water  issues. With a tank that small, you will want to do 25% water changes daily, with a 100% water change once a week. I also notice what looks like the beginning of fin rot. This will happen if the water quality is not too, there is no heater, and uneaten food is not removed promptly. A Halfmoon Betta's tail should be completely round with no jagged edges or peeks and drops in the tail and dorsal fin, which is the fin on top of his body. Without the heater, the fin rot will come back so it is very important that he has one. I think that once his home is correctly made, you will see a very happy and healthy Betta.