I bought my original 6' tank used and it came with a FX5 and a powerhead. (Maxi jet 900) By the time I got it going, I ended up taking the powerhead out cause it looked like it was causing too much current for my Mbunas, but no real problems. It probably bothered me more than them. But I would suggest one with less power than mine. But who knows, it might have been "tinkered" with, as the previous owner knew his way around electronic motors and such.....
My fish "seem" content without it. But I think I'm going to put a "bubble bar" in each end of the tank to make sure there is no dead spots. I also want to put back the one in the middle of the tank. But I like the bubble bars made from the airstone material. And I'be been trying to find ways to break up the tank and sight lines. It seems the more I add, the happier they look, with more males claiming territory/coloring up, and the less picked on the other fish are. 😁
