there is more than enough room for a filter and heater. they make small 25 watt heaters that will fit great in there. and filters cant have strong outflow as bettas (especially if they have long fins) will get blown around by the current. you can make a sponge filter out of an air pump, airline tubing, and old fish food container(or other small container) and some filter sponges. they have no current and do a great job!