Planted Tank Guru
Join Date: Jun 2004
Location: Baton Rouge, LA
That is Green Spot Algae (GSA). You can see the green color on the larger spots, even though some of it looks brown.
If this is a glass aquarium, a credit card or a razor scraper remove it fairly well. If acrylic, the Original (not any other) variety of Mr. Clean Magic Eraser works best. I have no idea if they sell that down under.
GSA appears when phosphate is limited. PMDD has no phosphate. It's a old fertilizing system, and it's been years since I've seen anyone using it. These days, reds are usually improved by limiting nitrate rather than phosphate, just enough to get the desired effect.