Get a Pondmaster (brand) model 1250 (or bigger).

It has a flow rate of 250 gallons per hour (with a fresh, un-clogged filter) and is rated to be used in a pond or tank up to 600 gallons. (When you have snapping turtles, though, that tank-size rating is optimistic.) I have a model 1250 filter and pump in a 175-gallon tank, and it is just right. Your 200-gallon tank should be fine with it, too. I don't use the fine-mesh carbon filter pad (which is for dissolved gasses and chemicals), only the coarse-fiber one, which does a good job at trapping debris (fecal matter, etc.).

The pump has a magnetic drive, so there is no motor burnout when the filter gets clogged up and the motor sucks almost no water through. I turn off the pump and take out the filter and hose it all down whenever the water flow is noticeably slower. I drain and rinse out the tank and refill it with fresh water every two or three months or so, or if the water gets noticeably stained from leaves or leftover food, etc. (my tank is outside). I point the output from the pump to a vertical position, so it sets up milder water currents (turtles prefer still water).

To hibernate the turtles over the winter, I fill the tank almost to the top and add a small bubbler and a 30-watt aquarium heater to the setup. The heater, along with the water pump (which remains on all winter), add just enough heat to keep the water from freezing all the way through (though it often freezes over on top for several weeks at a time). In fact, the water pump adds enough heat on its own that sometimes I turn the heater off for a day or two. (Ideally, you want the water to stay in the high 30-degree/low 40-degree range for the winter.) The bubbler helps the turtles with their cloacal breathing during hibernation.

This type of setup has worked for me for more than 10 years, with all kinds and sizes of turtles, including big snappers of 45 pounds.
 

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