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RE: How much current is enough?

Gordon Woodward wrote:

"Also on a related problem, I'm not particularly happy with the performance
the powerhead as the water current it generates is not nearly as strong as I
thought it would be. The specifications for the Powerhead says it has a
flow-rate of up to 1500L/hr but when used in my tank (4ft  x 18" x 18"), by
time the flow hits the other end of the tank the flow is nearly non existent
(with or without the reactor attached). I saw a demonstration of a Eheim
pump (model no. 1048) which has a pump output rate of 1200L/hr and it was
creating a greater current of water movement then my powerhead, despite the
discrepancies in output performance."

Gordon, I can vouch for the Eheim pumps rated for 1200 L/hr. I have 2 at
opposite ends of my 130 Gal tank (72" long). They have turned the tank into
a quickly flowing brook. To tell the truth, there is too much current,
although most of the fish seem to love it. Some of the plants could do with
a little less turbulence. I'm almost ready to totally re-do the tank, and
they won't be part of the new set-up.

Dupla recommended high circulation in The Optimum Aquarium, and to a point
it is a vaild argument, but there is such a thing as too much of a good
thing. Dupla recommends between 1-5 turnovers per hour (this is for
circulation, not filtration). In Aquarium Ecology, a Supplementary Approach
(a booklet put out by the German manufacturer Tunze), they recommend 1-4
turnovers per hour in a freshwater tank (and 4 - 10 in a saltwater tank).
Tunze goes further than Dupla in one aspect - they caution against the use
of too strong a pump, which can cause "hard currents" (pumps which produce
more than 2m of water column head). Shearing forces caused by such a strong
flow could damage or kill planktonic micro-organisms (there is planktonic
life in freshwater tanks too).

[I wonder what kind of shearing force would be created by using a "spray
bar" to moderate the output of a powerful pump - I think it might increase
the problem...]

Rather than using one large pump to produce the desired circulation, it
would be better to use two or more smaller units to ensure that although the
circulation is through, it is not so violent anywhere in the system that it
kills anything.

James Purchase