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On Fri, 21 Apr 2000, Tom Barr wrote:
Wow Tom, so many ideas!
> Something doesn't jibe here.
> There is some flow through a gravel bed, like Amano's or my Flourite/turface
> etc. These have very high surface areas and no added flow. These are some
> of the "best" substrates around and have no flow but due to the surface
> area/grain size and ability to have water circulate well through them they
> do a super job with or without a filter.
Coarse substrates allow some circulation, but without UGF or RUGF the rate
of circulation is very slow. Diffusion from the surface of the substrate,
mixing by burrowing animals, movement through plant roots, and maybe a few
other things all contribute to the circulation. In my tanks - and I
suspect in most tanks with coarse substrates - these influences are enough
to keep at least some areas of a substrate aerated to a depth of 2-3
inches. The actual rate of flow is something else.
The movement probably doesn't amount to more than a cm/hour, and a tank
volume will circulate through the substrate maybe once/week, or
once/month. In the rest of the tank the water will circulate over open
surfaces at the rate of a cm/second or so. In the narrow passages in a
biological filter the water will pass over surfaces at the rate of 10s of
cm/second. Comparatively, there's rather little opportunity for the
bacteria in the substrate to effect conditions in the water.
> What about all
> that mulm? It has always been a great planting additive for a new tank and
> an old one. It builds up and keeps on building up and the tank seems to be
> better balanced. What all is in that? Bacteria and inorganic matter I would
> think. What about rockwool? Does water flow well through there? Again, lots
> of surface area. There is a good flow but nothing like the filter or the
> water column but dismissing it as not a factor is simplistic at best.
These factors are all very important to the conditions in the substrate,
and so to the plants rooted in the substrate, but since there is only
limited water exchange between the substrate and the water column it makes
rather little difference to the conditions in the water column. I'm not
saying it makes no difference, just that it makes less difference than