04-19-2011, 08:17 PM
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Ok, I finally figured out the answer. Took me a while to find the Eheim manual on-line. The third line is a "breather" line that should not be inserted under the water (see your instruction manual!). Apparently this particular model pump works like a wet/dry filter and "pulses" the water through the pump. It alternately submerges the media and then uncovers it again to increase oxygenation. You can make the pump behave like a typical canister if you plug that smaller tube. I suppose the advantage of the pulsing behavior is to suck air in the breather tube and increase oxygenation of the media, just like a traditional wet/dry filter.
The intake of air through this line is normal.
Welcome to the forum!
Last edited by DKRST; 04-19-2011 at 08:19 PM..