x2 what Mary said, I believe the rule of thumb is that a 55 gal is the minimum size recommended so that you can achieve a decent amount of dwell time. Now a 30 on your up-flow sand and gravel filter or springflow filter as Mary stated would be just fine. When it comes time for the upflow media, check out crushed oyster shells instead of the sand or springflow layer, I am very pleased with the results. And what a find!! You can never go wrong with free!!!