Fertilizers promote algae growth... If the plants aren't using them.
It's not as simple as that, but that is the basic idea. If there is something that is missing (light, CO2, a certain nutrient, etc) then the plants won't be able to use the fertlizers and algae will be more than happy to oblige and step in to use them instead. Search this board for fertilizers or supplements and you'll find a wealth of information on the subject.
- Sam P -
plantedtanker in limbo - all tanks currently in storage