first off a 5/15 filter isn't near big enough to keep a turtle tank clean , the filter cartrige will almost always be clogged as turtles are very dirty , causing your water level to overflow the filter, with as much as your going to have to change out filters I would suggest buying bulk filter media as it's alot cheaper than those cartridges the cost of a 3 pack of those filter cartridges would pay for a 5ftx12inch roll of filter media and you can make around 25 filters
Bulk Filter Media, Floss & Foam Aquarium Filter Media for Sale Online | PetSolutions
you will also need to clean the intake tube and propeller daily as the mud will clog and wear out the propeller fast
if your still having problems you can call united pet groups direct they have a very understanding and helpful customer service team 1 800 322 1266