From what I've seen/been told here, find a 3/4 or 1 HP 2 speed pump. You need to check the pump's output and make sure it doesn't exceed the capabilities of your filter, but if you haven't had problems with your current pump, I doubt downsizing is going to present new filtration issues.
Tell us more about your filter so you can get some better advice.
(FYI, I'm a total noob with NO practical knowledge. I'm just regurgitating what I've read over and over on this board.)
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