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I notice a lot of you go by the gph of a filter. I've always ruled it by the manufacture's recommended tank size (even though it's meant for fish), because two different designs with the same pump with run differently.
here's a handy chart

You can find your filter and compare it to other brands with a similar GPH and see the difference in recommended tank size. There's a big swing between some.

I'm having trouble now with a filter that I want to buy,here, because it doesn't tell me the recommended tank size. And 290gph is equivalent to an Eheim for ~160gallon tank or a Rena XP1 for a ~45 gallon tank.
Confusing at best, right?

Thats a helpful chart, thanks for posting it.

We've basically used 3X filtration power guideline since it's easier for everyone to remember. Some canister filters account for the type of media used which will reduce the GPH... but I think most of the info is really unscientific.

No problem, Steve. It can be a headache. The filter I linked above, that I just ordered, has only a gph of 250. But it's rated for a 150g tank. Best I can guess is it's shear size. The measurements are huge, in fact I had to call and re-order the next size down from the biggest, when I measured and discovered it wouldn't fit under the stand!
http://www.jbjlighting.com/prod_reaction.html

The measurement that I use is 5 times the volume of water in you tank. I have gone through fish measurement times 2 but that is not always accurate. When buying a filter pump it is easy to not buy one large enough because it was sized for something close to double the fish tank. I have a 29 gallon tank with 300 gallons per hour flow. I have another 55 gallon tank and use 350 gallons per hour. The suction is under gravel and the discharged is deflected at one end.
By having a little more volume you don’t have to worry about slowing your flow down with the carbon, sponges and or ceramic rings.

Mke -- The fish people use one set of criteria for their fish and that does not equate for the turtles. Most fish people would love to sell you a filter that pumps twice as much as the fish size. But, even that is not big enough when you are talking about turtles. They are messy eaters and poo a LOT more and you have the scutes to contend with. Like your self I believe in the 5 to 1 gph. The more; the better.- If I could get away with it, I would have a small blender in there. ha