Bubbles in Filter Headbox: Air Entrainment, Not Leaks

Small bubbles rising from under lava rock in a filter headbox after startup — recurring annually — are almost always harmless air entrainment, not a leak or failure.

Common Causes

• New media off-gassing: Lava rock, bio-balls, or matting release trapped air for days/weeks.
• Pump vortexing: If pump inlet is near surface or flow is turbulent, air gets pulled in.
• Waterfall splash: Falling water entrains air → carried into filter chamber.
• Temperature change: Cold water holds more gas; warming releases micro-bubbles.

Diagnosis Steps

1. Check pump inlet: Fully submerged? No vortex? Add anti-vortex plate if needed.
2. Inspect plumbing: No loose fittings, cracked unions, or suction-side leaks.
3. Observe timing: Bubbles at startup only? → Off-gassing. Continuous? → Air leak.
4. Test flow: Reduced flow suggests air lock — bleed air from filter top.

Fixes

• Wait it out: New media bubbles stop in 1–3 weeks.
• Bleed air: Open filter vent/air relief valve daily for first week.
• Lower pump inlet or add submerged intake extension.
• Reposition lava rock: Ensure water flows through, not around, media.

When to Worry

• Flow drops >20% and doesn't recover.
• Pump noisy (growling = cavitation).
• Bubbles smell sulfidic (rotten egg) → anaerobic zones.

95% of headbox bubbles are cosmetic. If flow is strong and pump quiet, ignore them. They're just the filter "burping" after winter.