Loss of suction

I've had my tank running for 8+ years, and have never had this problem...until now. My overflow will lose suction. The pump then empties the sump into the tank up to the brim and on the vergue of overflowing the contents of the sump.

I noticed the slots of the overflow had a wear line or a line where the coraline algae was, and that's where the water line was, but no longer. Its over the mark by 1/2 inch...yep way over the line. Seems about once a week the syphon quits, tank pumps to the top and I have to restart the syphon...even then it can stop a few minutes or a day later.

I took the U tube off cleaned it..no help. Any thoughts would be appreciated.

 

Do you have a pre-filter sponge in the overflow? Maybe that is dirty and clogged?

 

No prefilter...removed it when this began a couple of weeks ago thinking the same thing...

 

It could be that the pipe or tubing from the pump to the return has calcified so that the output of the pump is much lower. Thus, the air bubbles that are in the U-tube no longer are forced down and collect resulting in a large air bubble which will cause you to lose the siphon.