Hydor's filters come in at the same £ as the eden and by all accounts are very good, if you have more money or don't fancy trying something relatively untested there's always the fluval 105 and eheim 2211 and 2213. Just about any filter with a 300lph bench mark will be adequate for most applications. My cube is 5 gallons and I would say 5-6 gallons start considering 600-700lph filters such as the tetratecs and larger fluvals.