Its got a filter but i think its a bit powerfull soo i will have to by another one. no i dont have a heater but it is near my bettas tank and there in a warm room.
You should Not put a guppy alone because it might be like this: the next day because it is too lonely. Guppies are schooling fish and are more comfortable in groups of 4 or more.
P.S.
(Don't even think about putting 4 guppies in a 2.5 gallon tank)
While Guppies are indeed schooling fish, they don't require schools like some other fish. Still, in a 2.5 gallon I would go with a nice male Betta instead.