I have 2 900s on a 30 gallon long (36"x12"x16") and I wouldn't want anything smaller. When I first put them in the tank was empty except for the sand and they were moving the sand around and I thought they were too big. But once the LR
was in there the flow seems about right, certainly not excessive unless you're right in front of the outlets.
Here's a pic of mine: you can just see the surface agitation in the middle of the tank at the surface