My son is looking to buy either a 50g or a 100g Rubbermaid stock tank for his turtles. I was thinking a pond filter would be best for this setup. Can anyone recommend an economical but decent pond filter?
Id suggest a DIY pond filter. You can get a 500 gph pump for about 50, and build a 20 gallon trash can filter for about 50-100 depending on what stuff you have laying around. Theres plenty of info on building one. A canister filter may work to but you want to go as big as possible due to the about of waste a turtle would produce. A DIY pond filter will be cheaper but a canister will but easier.
If the pond is large enough (100+ gallons) go with a small pond filter that has a backwash. This will be a lifesaver. You will simultaneously drain the pond for a water change and clean the filter (you won't have to drag out the filter, open it up, clean out all the crud, etc., it does it for you!).