what causes this?

[JoeA]

My tank specs are:
33gal long w/20gal long sump
all plumbing is 3/4" pvc, mag 7 submersible pump, HOB bak-pak skimmer.

Ok my return to the tank has a 3/4" elbow and the problem is that it looks like it blow either micro bubbles or it could be sand. I dunno really what exactly it is but I can hear the pump sorta puff and out comes a puff of this sand/whatever it is. I have no sand in the sump.

??

 

[fishfreek]

sounds like air bubbles in the pump. This likly a cause because the pump is cavataing due to restriction on the pump intake or because air bubbles are going into the intake because of the existance of bubbles in the sump.

 

[bound_for_obx]

This likly a cause because the pump is cavataing due to restriction on the pump intake or because air bubbles are going into the intake because of the existance of bubbles in the sump.

If your pump is cavitating I would address the problem ASAP. Cavitation happens when vacuum(or air) pockets develop near the surface of the impeller. When these pockets collapse the water acts like a mini jack hammer eroding the surface of the impeller away. If the impeller blades weaken enough they can break off and cause irreversible damage to your pump that might mean replacement of the pump. I have seen pool pump impellers eroded away to almost nothing. Like fishfreek said, look for a restriction on the intake side of the pump, or bubbles entering the pump.

 

[JoeA]

what I noticed was 2 things...one the flow of water from my over flow box is producing bubbles in my intake chamber. The second thing was if i closed the valve on the return from my mag7 and allowed the water level to raise in the return chamber the bubbles seemed to be significally less. I constructed the sump with 2 bubble traps one on each side of my Refugium. I do haveaball valve on the overflow box side but would rather not use it ...i'm afraid it could get jammed and flood.

 

[Brad]

Is it possible that there is a pinhole in the return PVC line? Any pinhole in the joints of the PVC return will act like a venturi, sucking very small quantities of air in as the water rushes by.

Test your joints dripping some water over the joints as it is running. If this causes any momentary change in the bubbles shooting out, you've found the leak.