I also tend to live on the edge with algae but I also look at it being coming more than just introduced by what I move in. I feel lots of it are just blowing in the wind and will find the tank. But what happens at that point is where I deal with it. I have a tank which is often set in bright light in the window and that tank often gets plants, fish, stuff moved in from other tanks. But when the "stuff" from other tanks moves to the bright sunny tank, things often explode and algae is one of them. I think of it more like the cold or flu, in that it is always there, just how the tank is suiting for it to take hold. Fight it quick and early and in the right way and it goes away but let it get a good start by treating in the wrong way and it will fight you to the death!