Not sure on the actual flow. Pump is the Varios 6 which has a max flow of 1720 gph. I've currently got it running at 53%. Tank is 5'long x 2' wide x21' tall.For the RFG to work, it does need to be fully submerged. And the more flow you push through it the deeper it would need to be to avoid drawing in air. If you can provide an estimated GPH you plan on providing each nozzle, we should be able to give you an estimated depth you need to have he RFG at.
Also -what are the overall dimension of the tank?