I think there is some confusion here.
While there may be reconditioned, continuous run compressors available, my compressor has a 20 gallon tank. It is constantly bleeding off air and the bubble is continuous as long as the air tank has some pressure. The compressor pump only kicks on for about one minute to COMPLETELY fill the 20 gallon tank (from the bubbler continuous flow) about every 30 minutes.
As for the regular, continuous run compressors, when I checked for the size pond I have, and to run the amount of air I need to, the prices I saw were > $1K and some as high as $2.5K.
Do you have a link to the ebay pumps you are referring to? If they are that cheap then of course buying a $300 compressor doesn't make sense (even though I already had mine, and the air hose).
