Have your phosphates checked with a digital Hanna meter. NSW has <= .02ppm phosphates. High nitrates, phosphates, and light is all algae needs to become a problem. If you can control these three things you can defeat the algae.

Phosphate comes from fish food(preservatives), can be leached out of Liverock and sand beds, and can be found in tap water.

Using phosphate remover (GFO, phosguard, chemi-pure elite, phosban, etc) is a very effective way of lowering po4 to proper levels.

I use GFO HC from BRS along weekly water changes to control nitrate and phosphate.

HTH

Louis