Bettas need a weaker filter. Yours sounds fine. As for a heater, I'd be concerned for the betta and the tetras. Currently tetras are growing weaker in general because of breeding practices and whatnot. They aren't as hardy today as they once were. They should have a heater as it will lessen their chances of common tetra diseases and lengthen their lifespan. The betta will do well with one too. If the tank water gets as cold or colder than the house then your fish aren't digesting much and will be weaker also. Most tropical fish start to shut down their metabolism at that temp.