Hello Tank...
Just get a couple of Hagen AquaClear HOBs with a combined gph (gallon per hour) rating about 6 times the volume of your tank. In your case, you need a turnover of 450 gallons of tank water every hour. Two AC 70s is plenty. That will give you a gph of 600. Just position them at opposites ends of the tank. Pretty simple.
Or you can get some Chinese Evergreen plants that thrive emersed in the tank water. They'll remove the pollutants in the tank during the day and you can use a single AC 70 to do the job at night when the plants rest.
Attached is a pic of a 45 G with the set up described above. You could do the same with your 75 G, you'd just need more plants.
Just a thought.
B