First off, is it 110 or 12 volt. If it's 110 then you are really limited to your choices. You might be able to find something with more blades that turns slower.
If it's a 12 volt or lower then you can probably find a good computer fan.
I had the same problem on mine (110) and never did find a suitable replacement.
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250 gal display tank plumbed directly to my wallet
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