"If you choose to use a canister filter or any filter, no matter how many stages; biological filtration (the nitrogen cycling bacteria) is still the most important function. Maintaining a balanced, established bacterial load via contistant regular partial water changes is the key to maintaining water quality, not how much the filter cost or the flow (GPH) because water will only hold so much dissolved oxygen and that is inversely related to temperature and the bacteria can only utilize so much dissolved oxygen to redox ammonia and that is temp. dependent. And while carbon makes the water look pretty for a while it will be difficult to keep up in a small volume pond with the organics load of turtle turds."
Jeff,
While all of what you said has truth to it, I think there's a misapplication in your thought process. There's a big difference, I think, in determining how well one's water quality is for a fish aquarium, water turtle setup or koi or plant pond as oppossed to a simple pond for box turtles.
What you listed about biological filtration and such is absolutely, spot on, true if keeping fish or water turtles. But seeing as this is for box turtles, I think my answers to him were based purely on the facts of what he said he had and wanted and the fact that I was concentrating on keeping the water "filtered," as in clean, not as in well-balanced, in the common sense for fish. Don't get me wrong, what you list is all very, very true. But I just think it's a little too much to worry about for a box turtle that does not rely on water for anything else than to drink and soak in.
To your point about water flow, my comments weren't based on how much oxygen the water could hold, etc., but rather that a proper flow, not to torrent and not too weak, would be best for the filtering off of large debris for his sized application. Also, rest assurd, carbon does a lot more than make water "look pretty." It's a fantastic purifier, can remove chemicals, metals, and other harmful contaiminents that common water testing can't account for or answer. I don't mean to say all this to start a fight, I just felt you might have been overthinking things a bit for this guy's situation. Having worked in the aquarium trade and serviced and maintained systems from freshwater to saltwater to ponds to reefs for six years, I know my stuff too. 
And by the way, I completely agree, MMathis, it's far easier and efficient to just keep a large dish or pool of water and do routine water changes.
Jeff, here's a pic of my saltwater, as a peace offering.
