I have been plauged with cyanobacteria since replacing the bulb in my metal halide light unit.

I tried the 3 day blackout which worked very well, unfortunately the cyanobacteria returned and started to spread quite quickly. I didn't want to put the plants / fish through another blackout. After trying to obtain erythromycin to no avail (uk law). I added a product called Myxazin which is a bactericide. This has stopped the cyanobacteria in its tracks and it is now disapearing.

Unfortunately I cannot say with certainty that this is a magic cure as I have made other changes since the cyanobacteria appeared. However it is worth trying if all else fails.