Is this a good setup for a betta tank?

You dont even need a filter for bettas bc they dwell in small pockets of water naturally but make sure you change the water on the tank at least 25% once a week

A sponge filter is easier to take care of than an under gravel filter plate. With the sponge filter you will just rinse it out (in tank water that you have syphoned from the tank)and replace it but with the under gravel filter yo will have to clean under the filter plate and you have to take the gravel out to do that. I am glad tha yo have a heater in with your betta as bettas d better at near 80 degrees than just "room temperature" as room temp can be any where between 65 and up. I am surprised that bettas can survive in such low temps.

Hi Metallo; My son has a white betta in a 2.5 gallon tank in his room. It has an undergravel filter system and it works very well. It keeps the water moving at an even rate and keeps the tank clean. The betta was in a smaller tank and we just set him up in this new one last week. The fishie (his name is Frost BTW) seems a lot more active and happy. His doesn't have a micro heater, but he does have a thermometer that we check on a regular basis. Just make sure that yours is not placed where it can get direct sunlight and it should be fine. Good luck. :)