U Tube

[liquorfish]

About every 3 days the U tube in my pre-filter gets air in the
top of the tube. Why?
I siphon the air out and little by little it's back.
Decreasing or increasing the flow from the fuge did'nt change
anything.

 

[reefnut]

How far does the U-Tube go down in the overflow? When water flows over into the overflow sometimes it produces bubbles. If the U-Tube isn't far enough down it could catch the bubble and cause air to get in... an increased flow should help push the bubbles through though.