what hang on the back skimmer should I go with

I have seen the same Remoras moved from one tank that it was not pulling much stuff out of the water and in to another tank and it pulled a tun of stuff out. I don't know why or what the difference was but it was the same skimmer in both tanks I saw it being transfered from one to the other. The next day it was pulling some darkish green skim out and it would only pull a light tea skim out of the first tank. I do not know why it was working so differently in the two tanks. The two tanks had similar bio loads one had a bit more in the way of coral than the other and the other had a couple more fish.
So what is so different to make the same skimmer work so much better?

Bill