Canister Filter Positioning Question

[Sysiphus]

I get a bubble flow from my fluval (it is about 3 feet below the tank surface level) on occasion and the cause is always cavatation caused by flow restriction of the intake. For me that restriction is almost always accumulated plant material partially blocking the intake. So just an observation - be sure the screen on the intake is clear - fluval have an anti-syphon gizmo (technincal term) and my obstructions are seldom visible and I have to wash the intake screen and anti-syphon valve very completely to clear it.