I would have thought at least 650l, but prob a bit bigger. I have a 500l tank for my own 1 man setup, and i reckon i could manage with 350l, but no less. I have read on this and other(!) forums about problems using 1 pump and 1 controller. As far as i can see, the only way round this would be to use 2 pumps and 2 controllers which could be done, but obviously you would need 2 outlets on your tank. If i were to set up a 2 man system i would use 2 separate 350l tanks (upright) within the same frame, each with their own pump and controller.