A big spot on my puffer (and it's not a spotted puffer!)

[jassz]

So why is it a concern that this type of algae die and release PO4 and Nitrate, but not other types of algae? What makes some good to have and not others? And why would I have to put chaeto in the sump? Why couldn't it go in the tank (I don't have a light in my sump, and hte skimmer and sump take up 2 sections and the third is where the water drains back down so it's like constantly running water).

Thanks for all this information, BTW. I really want to understand what I'm doing.