I have used it for cyanobacteria with success, however, many hobbyists have claimed that it has crashed their systems. If you have cyano, you may want to exhaust other alternatives before going with chemicals. You must not under any circumstance overdose or over use chemiclean.
I dosed Chemi-Clean last weekend, then did a 25% water change. It's only the second time I've used it, but I've had no loss of life in either instance. Copepods, amphipods, snails, crabs, shrimp, fish, and corals have shown no response to the treatments. I slightly under-dosed both times, and it worked. Will make your skimmer go nuts for a little while. Make sure you turn it way down or it'll blow the top off with all the excessive foam. Mine settled down after several hours, but it can vary. I just checked on it every couple of hours for the first day, and it was fine after that.