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>Wow Tom, so many ideas!

Oh, I'm just getting warmed up. I don't send but maybe 20% to the APD which
may keep some folks happy! Thanks, but you are just as guilty if not more
>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.

But aren't bacteria's needs very small too? They may just take a
proportional amount longer to react than the filter bacteria? And this
slower rate adds to the cycle in slow small amounts which is good for the
plants and keeps the algae at bay? Wouldn't a shock to a tank like this (no
filter) take longer to rebound than a tank with a plant or bacterial filter

>> 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. 
>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
>other factors.

OK, now what about those pesky RFUG's I use? They have lots of flow almost
to the point of fluidized bed filters. And they grow plants very well like
no flow substrates. Then there's the heating cables. These act as both
filter and substrate.

Tom Barr