I just got through an ugly cyano outbreak in my 75, it was covering the rocks and sand. I siphoned out what I could with my waterchange and made sure the flow was ok everywhere. Took about a month but gone. As for Powerhead placement I always have one in the center of each end panel as high up as I can without them sucking air and alternating slowly with just enough power so the sand doesn't blow all over from the undertow.
