I would suggest 2 50 watt heaters for it you could get 2 100 watts and fit it in , the heaters I use are only 6 inches long the 100 is about 7.5 inches long and still fit in to the filter box Catalina Aquarium Titanium Heater
these are the heaters that I use ..
I also took the biowheels off and tossed them
after the inital cycle they are nitrAte , phosphate factories . I never had them on the filter