Large, external filters can seem expensive, but are the best system to keep your tank clean. Always remove all media from the filter such as carbon. Only use some form of bio-balls and coarse sponge as filter media. (Filter wool is fine but will clog quickly). Ammonia removers, carbon etc quickly loose their ability to work effectively and begin to leach toxins back into the turtle tankwhich can lead to illness over time.
Never underestimate how dirty turtles can be. You need to change up to half your tank water weekly and all your water monthly. Internal filters need weekly cleaning with your water change. External; filters should not need cleaning for up to 3 mths or more depending on size of tank, amount of turtles etc. Place a 'pre-filter' sponge over the intake hose to decrease the waste matter going into your filter. eg/ cable-tie a coarse sponge around the inlet filter and remove it for cleaning frequently. This stops loads of bio waste getting inside your filter.
NOTE: some turtles will eat the sponge so you cant use it for those turtles. Remember that for every poo, turtles do 5 times the amount in wee, so change some of your water every week so they arent drinking and swimming in their own urine.