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Re: Filter capacity

kristiananderson asked :

> Does anyone have a recommended filter flow rate (GPH) for
> a 29g planted tank?  

A very wide varitety of flow rates will work for a wide
variety of conditions.  A lot of different folks recommend
different things, anywhere from one or two times the tank
size/hour to five or more times the tank size per hour.  

Rule of thumb: very little is needed (maybe 1 times tank
size/hr) but it's hard to have too much (4 or 6 times tank
size/hour is fine if your fish don't mind the current). 
I'm taling actual flow rates, not the exagerrated ratings
on the filter box.  For a 29g, a filter flow rated at 60
gph rated pump (which actually pumps about 30 gph) or so
will work but so will an Eheim 2026 which is rated for 250
gph (and actually puts out about 100 -150 gph or so most of
the time).  Unfortunately, most filters with a 60 gph flow
rate are going to be pretty small and the media may need to
be changed very often (maybe even daily or weekly, ugh).

A wide range will work fine for most conditions provided
that the other important filter considerations are taken
into account such as:

The amount of filter media -- Eheims tend to use relatively
large amounts of media with relatively low water flows
compared to some other brands.  The less the media, the
sooner it can mechanically load up and the less biological
activity it can support.  A real high flow rate with a very
small amount of media won't get you a lot of filtration.  A
small mechanical filter will load quickly so put that
medium where you can get at it easily (like a sponge sock
on a canister intake or foam block in an wet/dry overflow
or sump).

How completely the water in the tank circulates -- In a
heavily planted tank, you need enough current to keep
nutrients moving around and available to all the leaves --
and the oxygenated water moving around for the fish.  But
that doesn't take three or four times the tank size/hour to
accomplish! It requires thoughtful placement of the intakes
and outputs and a very gentle current can be just fine.  In
a fish-loaded (or overloaded) tank, you might need to rely
on a lot of mechanical filtration and bio-filtration, along
with water changes, and you'll regret that your 3 times
tank size/hr filter just doesn't seem to keep up with all
that food you're dumping into the tank.

What kind of media/filter you use -- or so it seems.  It
seems most folks with wet/dry set-ups like to have
monsterously large water flow rates.  The surface area per
gallon of bioballs is very small compared to some other
kinds of media used in canisters.  (Otoh, bioballs don't
clog, they slough.)  But I've always thought that high flow
rates had more to do with getting the overflow and sump
levels (intake and discharge rates) balanced than with
actual filtering needs.  Also, imo,  lot of the
recommendations regarding wet/dry filters are inherited
from unplanted tanks and reefers.  Extremely porous
materials like eheim ehfisubstrat, lava rock, or SeaChem's
Biomatrix may beome less porous over time as biofilm and
various curds accumulate and fill the pores.  Otoh, SeaChem
states, "Matrix has a porosity large enough to allow for
the washing out of any dead bacteria, so that is not an
issue. As far as the biofilm is concerned, it is normal to
have a biofilm on the external surface area  where the
aerobic bacteria flourish)" 

Whatever pump or filter you get, count on the *actual* flow
rate to be roughly half what's rated.  Biofilm in tubing
and on the pump impeller and rotor, bends and joints in the
water circuit, filter media -- these things restrict the
flow.  There's a lot of math that you can do to calculate
just what all these effects will be (a sharp elbow costs
5%, a joint costs 2%, e.g. see _Aquatic Systems
Engineering: Devices and How They Function_
by Pedro Ramon Escobal, Martin A., Jr. Moe) but when you're
done with all that work, you'll end up a with an estimate
that still based on lots of assumptions.  So just figure
half and you'll do fine.

Scott H.

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