If you only have one powerhead and that's your total water movement I'd suggest that it's not nearly enough. You could add a couple of maxijets, one on each end pointing towards each other. As Josh said, a cyano outbreak 3 months into a new tank is pretty typical. My new 60 corner is coming up on 6 weeks and I'm seeing some cyano, that's with no fish, mostly cured rock (it only cycled to .1ppm ammonia), a big skimmer, and a 1000gph return pump plus 2 maxijets.