If you get an Aquaclear, don't get one smaller than the Aquaclear 70--the 50 isn't powerful enough for a 20-gallon; the 110, however, is way to powerful.
Big al's is a favorite place of mine, but you might want to check a few other places than big al's for Aquaclears---they have increased prices for these filters significantly. The big al's rep I spoke with said that Hagen has made them do this, mainly to make them more on par with retail stores, which would help increase the latter's business(!) The Aquaclear 70 now sells for $41.29, which is more than I paid for a 110 ($35.99). The Aquaclear 110 now sells for $61 and change, a hefty increase.
A turtle will still get some UVB rays if the holes in the screen are less than 1/2 inch. You wouldn't want to have a screen with holes as small as 1/8 of an inch, though---this would screen out the vast majority of the UVB rays.
