Posted: Sun Nov 09, 2008 2:55 am
Since 3 inches of turtle isn't that much bioload, if you do two of the diy canister in parallel powered by a 300 gph pump it ought to be sufficient. Just make sure to put good biomedia in! That's the most important part.
Also, to reduce the frequency of required water changes, you might try to figure out how to put plants in the tank so that they won't get eaten by the turtles.
-robind