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.

I would imagine it doesn't discriminate between helpful and harmful bacteria so be sure to keep an eye on ammonia levels, etc to help you stay on top of the condition of your biological filtration, just in case

Whats your tank setup? Co2? dosing?

The reason it keeps coming back is that the cause of it hasnt been changed.. treating the symptoms isnt going to help you for very long.