Why isn't my Remora working?

[littletyger]

This tank had been up and running for 2+ yrs. In June I moved the tank from home to work and changed the pump (Maxijet 1200), but it was a used pump. It was working fine. Then 2 or 3 wks ago I noticed that wasn't producing any skimate and that there was hardly any water coming out of it. I took apart the pump and cleaned it, but it didn't change anything. I took a week off from work and when I got back there was red slime algae in the tank. I got a new MJ 1200 and put that in. It's been a week, and now it has more water coming through it, but it's hardly producing any bubbles. I can see bubbles underneath the collection cup, but that's all. Any thoughts?