Picking a filter

Buy an inexpensive and simple filter that keeps the charcoal and foam separated and then-- leave out the charcoal.  You don't really need it.  You can use an external or internal filter. I really like the top fin internal filters at Petsmart .

With good quality coarse foam, you should be able to rinse and reuse it for ages.  No more buying filter cartridges and charcoal messes.  Once a week, rinse the filter in a bucket of tank water.