Overflow Flucuation

[Spooky]

Occassionally the water in my overflow rises and falls about an inch or so. It happens in a very steady way much like the coming and going of the tide. Usually it only happens when my sump has gotten low and the return pump is close to the surface. I should also mention that my overflow has a lot of algae growing in it as I didn't cover the glass on the back of the tank where the overflow is and the tank is in front of a window (the rest of the back is painted black). I am putting a piece of paper over the unpainted glass tomorrow to block the sun and also plan on manually removing as much algae as possible. Has anyone else experienced this rising and falling water in the overflow?
Thanks in advance
Spooky

 

[penguinsix]

It is normally from too much or too little air getting into the overflow. If you can put a valve to limit the air intake you should be all set once you find the sweet spot.