I use the Dart on my 300G tank. If you are just using it for a return, you don't need the pressure pumps. Why do you think you'll need 5000GPH? If you are planning on running a lot of other things off it, that's fine. If it's just for return, remember that's a lot of water going through your sump and you'll probably have microbubble issues. For additional flow inside the tank, use powerheads.