Here are a couple of more thoughts: the filter and pumping system is designed for clean water and clog very easily. Consider plumbing the outlet of your external filter into the intake of your tub filter thereby "prefiltering" the water entering your tub's filter and heater and still being able to use the air pump in the tub for aeration. If that proves to be too difficult you may also consider bypassing the tub pumping system completely and simply mounting the heater to the outflow of your external filter and then plumbing a line from your air pump through a hose to a large airstone.
