What filter is best for a ten gallon saltwater aquarium?

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For a mechanical filtration an AquaClear 20 would run enough gph for the tank. Also the media basket is great for things like activated carbon, Purigen, etc. Don't use the sponge or ceramic media, that's what traps detritus. You'll want to clean it once a week with your water changes. You could also turn it into a mini-refugium. Technically though, all you need is a 100gph powerhead pump and at least 5lbs good live rock. The rock will act as media for the beneficial bacteria. The deeper pores within the rock will also aid in denitrification, which is critical for a healthy marine system.