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slow UGF

I don't know what an optimal flow rate would be through the substrate. I
think I based it on an estimate provided third hand by Karen R which was
something like 24 hours or more for water to move from the substrate
surface down to the bottom. Even that seems very high to me now since it
would induce quite a lot of oxygen into a relatively narrow area of the
substrate. I don't remember the basis for the estimate. Perhaps it was
derived from estimates of ground water movement? These vary with

There was a discussion of this very idea in TOA in which the authors
discarded the idea as impractical. They decided that you could not
produce a uniform flow via an UG plenum since the water always seeks the
path of least resistance and 95% of the flow would pass through a very
small area. Still, if you have a pressure differential you will still
have some slow motion of water through the grains of the substrate in
the other zones and it will be much lower than in the primary flow zone.
I think the idea would be to have it as slow as possible.

If you are using the vacuum pressure outlet from a powerhead, this will
develop a vacuum of several inches hydrostatic. (i.e. the pressure
caused by a water level difference of that height)

And the purpose of this is to filter nutrients from the water? Plants do
this very effectively using biological mechanisms!!! The bonus is, its
very cheap and requires no special hardware! ;-)