We have high nitrates in the tap water and I don't usually have a problem with cyano, just the once when it came on infected plants.
Have a look at this. It is part of an algae article that will be in the next Aquarium World magazine...
BGA - Blue Green Algae, slime
This type of algae usually occurs from beneath the substrate, and rises up. You can visually see this happening before a breakout begins, so it always worth keeping an eye on the higher levels of the substrate.
BGA is actually not an algae but a mixture of Cyanobacteria & Algae. It grows in sheets and can have a bad smell to it.
Causes -
A) Low nitrate levels
B) Areas of poor circulation with low O2, such as the substrate
Solutions -
In my experience the best way to totally get rid of BGA is to perform a 3 day blackout on the tank. Doing this will stop the development of the algae, as no light will be entering the tank. Plants will do just fine with this, as they have a more complex system over algae that allows them to survive longer without the correct environment.
To perform a blackout it is very simple and risk free.
1) Cover the tank completely so no light gets in, i have used bin bags to cover small tanks as well as old rolls of wallpaper.. you get the idea.
2) Turn off any CO2 system that may be running.
3) Use an air pump when covered.
4) Do not feed during the 3 day blackout
5) Upon completion of the blackout, do a 20% water change. If the outbreak is really bad, it is worth checking the filter to remove any excess BGA that may be there.
In all the cases that I have done this, the algae has died totally, and the plants give a nice growth spurt when the lights and normal conditions are resumed.