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Re: Noisy canister filters

James wrote:

> I have an Eheim 2260 (the BIG one) that I use on my 130 gallon aquarium. I
> don't use it as a CO2 reactor. The intake for the filter is reliably under
> 18" of water at all times, yet it (the filter) burps and gurgles all the
> time. If I shake the unit, a large "burp" of trapped gas will come through
> the output tubing and gets released into the tank (it doesn't seem dangerous
> and it doesn't smell of sulphur). It has never dawned on me that air could
> be being drawn into the cannister through the o-ring seal (its been a while
> since I've replaced the o-ring and I don't use any petrolium jelly on the
> ring to seal any minor leaks).


I'm not familiar with your Eheim.  In fact, I'm barely familiar with
mine.  My Ehein 2028 as well as the Magnum 350 have the filter medium on
the suction side of the pump.  That means that pressures in the canister
are relatively low and may be less than atmospheric pressure.  This is a
good thing since low pressure inside the canister means that the
canister is unlikely to leak water out even if the seals are bad.  The
inlet tubing is also under low pressure -- possibly less pressure than
the canister itself.

A small leak on the tubing leading into the canister filter can cause
the filter to pull air through the leak.  Pressures change when the pump
is off, so seals that leak air in while the pump is on might leak water
out when the pump is off, and so would be easily found.  A small leak
may not pass enough water to be noticeable and a leak in a joint above
the water level in the tank won't leak water out.

The canister seal may be under suction or under slight pressure,
depending on details of the filter condition and design.  Most normally
it will be under suction.  In that case, a small leak in the canister
seal can cause the canister to draw air.  A large leak will probably
leak water when the pump is turned off.  A small leak may not.

I concluded that my leak was in the canister seal because I only had to
open and reseat the lid in order to stop the leak.  In severe cases the
seal would leak water when the pump was turned off.  It leaked quite a
bit of air, not just a few bubbles also the bubbles were inside the
canister, not in the outflow.  Sometimes I would find the canister half
filled with air only a couple hours after I closed it.  The Eheim, with
its pump at the top of the canister, probably won't let very much gas
accumulate before it clears.  That means that the Ehein shouldn't get
gas accumulations that are as big as they get in the Magnum.

I think that when the plants are bubbling and the gas is supercharged
with oxygen that the low pressures in the impeller can cause bubbles to
form as water passes through.  This is a different situation.  The
bubbles in the outflow create little or no noise and there isn't any
volume of gas in the canister.