How much flow do you want in the tank, and how long is the tank? I have a Koralia 4 on one end of an 8' tank, and it provides good flow for about half the length of the tank then peters out.
I think either a 3 or a 4 would be fine. Just be sure to break up the current in places with driftwood, rocks, etc. so the fish can choose how much current they wish to be in.
