[nrosdal]

I would say 2 will be plenty. I have one wp40 on my 4ft 120g and i get major flow more then 2 feet back after it hits the opposite side glass and flows back(running it dialed down 1 notch above lowest setting). So i would imagine that a 6ft straight shot would get plenty(especially if it is a fowlr).

Hope this helps. If you are curious there are tons of youtube vids people have made on that size tank.

Thanks