Most filters are rated for fish. Turtles are much more dirty(more poo, shedding, ect ect). Generally you want to triple the ratings on most filter(say you have a ten gallon tank you would want to get a 30 gallon filter).
All filters are going to get dirty relatively quickly, so it is always nice to get an easy to clean filter.
A nice easy to clean filter that I have had for over three mouths is the Zoo Meds Turtle Clean 318 filter. It has worked fine for my tank and not only dose it have a big foam pad, but it also has two compartments that can hold small amounts of custom media(I use carbon and bio).
You can also try the Turtle Clean 501 Canister Filter for practice with canister filters that you will most likely have to use in the future.
Here is the link to both of the filters above...
http://www.zoomed.com/html/filters.php
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RES's
Tank, SCL 3.75in.
Diesel, SCL 3in.