red algae

[Delphinus]

Well, if you get really desperate, there's Red Slime Remover, but you have to remember it's treating the symptoms, and not the cause. Without addressing the cause, it will come back, sooner or later.

In a nutshell, it's a nutrient load issue. Although it's not uncommon in new tanks say around the 6 month mark. Remember that for the first year there's all kinds of spikes and cycles (not just the ammonia cycle), before anything somewhat resembling a stable equilibrium is attained. But getting back to the nutrient load, basically, managing import vs. export. E.g., feeding, skimmers, water changes, or macro algae harvesting, and so on.

PS. Sean is right, cyanobacteria. If you search here and on RC you will find heaps of pages of information. Good luck!