As long as there is any sort of flow in the water then you dont need one. Using powerheads, pumps, wavemakers and or any sort of thing like that should get the job done. You just need to have it according to your tank size. Something like using a nano koralia in a 50g tank would not suit it. Just the same if you put a koralia #4 in a 15g.. It all needs to be according. If you ARE going to use a 20g long, then i would say get 1 korlia #1 for good circulation. But if you had a 20g high, i would think a nano koralia would work. Only because the 20g high has less length that the flow of the pump would need to reach. See what is in the budget, and try and go for a koralia. They are awesome.