You mentioned a "Seahorse Tank" - having any problems with it? If not, your water source probably isn't the problem.

I wouldn't "Cook LR" - seems that would only create "Dead Rock", which would just introduce more nutrients back into the tank.

Are you sure what you have is cyanobacteria? When I get cyano it is mostly on the sand/substrate and not on the rock. If on the rock, it easy is washed off with a turkey baster. Could be some other type of algae - can you describe or post a pic?