[Delphinus]

Not really an overstocking issue per se, but commonly suggested as such because the more mouths to feed means more food/nutrients which then leads to cyanobacteria. Generally speaking, a high incidence of nitrates is behind a cyanobacteria bloom although there can be other causes such as old lights (when I used normal output actinics I could tell nearly to the day when they hit six months old because the cyano would start, and disappear when I replaced them).

You can treat the tank with "Red Slime Remover" (or "Chemiclean" or other commercial cyanobacteria treatment) - it will turn your tank yellow for a day or so, and turn off your skimmer if you have one because it will go nuts - and then you need to do water changes plus run carbon after a day to clean the water out of the dead cyano, but it works like a charm. However it's only a symptoms treatment - if the cause of the cyano isn't found and addressed it will return sooner or later.