The longer the better... just remember it needs to be completely vertical. Another member used an FX5 and theirs was a little short, which caused some bubbles to make it to the tank. If you've got room, 2 feet would be sufficient. If not, bioballs might be a good idea. I don't have bioballs in mine, but I'm running an eheim 2224 on one, and a rena xp3 on the other.
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