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Old 11-16-2012, 05:53 PM   #8
Hoppy
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Any system that pumps tank water under the substrate, so it passes through the substrate back to the tank, is a under gravel filter. Your reasons for using it don't change what it does. The goal should be to get roughly equal flow from every hole in the tubing. That requires that the total area of the holes not be much greater than the cross section area of the inside of the tubing. If the total area of the holes is much less than the area of the tube ID, you get strong jets of water from each hole. If the total area is much more than that of the tube ID, you get slower jets, and the holes farthest from the inlet to the tube may have very little flow out. When I was using this type of filter I didn't notice any stirring up of the mulm on the substrate. The flow from the jets gets diffused so much by the gravel that you can't see anything happening.

If there isn't much gravel over an area of the jets, you get a miniature gravel volcano, which doesn't look good. So, it seems that you have a choice of no visible effect or a bunch of miniature volcanoes.
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