It's all about the rate of filtration, not the manufacturer's stated "size" for a filter. Take the gallons per hour (gph) and divided by the thank capacity. For example, I have 150 gph and 20 gallons. This makes 7.5 filtration cycles or "turns" per hour. I would recommend at least 4 to 5 turns per hour.
I agree with welchrock - you probably want to measure the actual flow rate of the filter and not just stick with the manufacturer's statement.
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