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Re: [APD] Water Circulation -- or - The Current Sponginess of Filtration Theory

Has anybody got any empirical info on the rates of tank
turnover?  I can't think of anything with more answers and
less data than this perfectly good question.

Answers range anywhere from 1/2 per hour to 5 to 10 ties
per hour. I suppose if one knew just how much food and
ferts were being added, the fishload, the plantmass, the
filter meachanical capapcity and filtration efficiency
(which changes at diff flow rates), filter
chemical/bacterial capapcity and filtration efficiency, the
extent that filtered water mixes with the unfiltered water
before being brought through the filter again, and half
dozen other things that are not likely to beknown or remain
constant, if one knew all that, then one could engineer an
optimum turnover rate. However, you'll rarely see that done
-- too much data missing for too many variables. And if you
do see it done, it won't be easily applicable to another
case without making appropriate adjustments in the values
of the variables, for which most of the data will be
unknown or dynamic.

Luckily, we have on hand the poor person's empiricism --
anecdotes, judgement, hands-on experience, eye-balling, all
of which says that it's one of the least fussy issues for a
planted tank.  Basically, if you've got current, you've
probably got enough.  And you probably won't have too much
unless you're creating turbulence and shedding CO2. So
there's an awfully wide range there. With a wet/dry, the
flow rate can have significant impacts on ratethat CO2 is
shed. But other than that, whatever you've got, that's the
right amount. And I've got the anecdotes and eye-balling to
back it up !  ;-)

Don't forget to throw into the equations that the rated
flow for most filters and pumps is not the flow that you
will have 99% of the time. Sometimes the rated flow assumes
nothing is attached (zero dynamic and static head);
sometimes they assume a perfectly clean filter media,
tubing, etc. And sometimes the rated flow is, well to put
it nicely, just plain whimsical, giving diff rates for the
very same pump or the extreme example, Eheim giving worse
rates for the same pump against higher resistance than

If you're set on calculating rates, here's a place where
you can look up some of the logic and math for determing
flow rates:

_Aquatic Systems Engineering: Devices and How They
Function_,  by Pedro Ramon Escobal,Dimension Engineering,

But you'll notice that Mr. Escobal handwaves away a lot of
the variables with blanket assumptions.

Or you can forget all that and worry instead about how
you're going to pay for the lights and CO2 system.
How much light? How much CO2?  Glad you asked -- things are
a little less spongey once you get away from filtration ;-)

--- Randy Pullen <RPullen at waterpik_com> wrote:
> I read that the filter system in a planted tank should
> only circulate
> half the volume of water in the tank per hour. What is
> the guide line
> for this?
> My 38 gallon has a reverse flow undergravel filter rated
> at 190 Gal/Hr
> and also a power filter turned down to lit's lowest  flow
> rate but is
> rated at 145 Gal/Hr This would be something like 8X the
> tank volume per
> hour.
> I am adding pressurized CO2, have about 18 small fish and
> a fairly well
> planted tank.
> Any suggestions on changing this or is this okay? I am
> struggling
> slightly with BGA on the substrate and lower stems of
> plants, sometimes
> I wonder if the reverse flow undergravel filter brings
> out phosphates or
> waste to the surface of the substrate?
> This configuration was suggested by my LFS that had nice
> planted
> tanks.

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