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Re: Aquatic Plants Digest V2 #1054
> Date: Sun, 2 Nov 1997 21:18:37 -0500 (EST)
> From: IDMiamiBob at aol_com
> Subject: Heating cables
> I'm confused about this whole heating cable thing. Perhaps this is partly
> just a conflict of approaches between differing schools of thought. I read
> on this list that UGFs are bad because they cause a current across the roots,
> which reduces something, maybe it was the develpment of root hairs. But
> others are fighting with heater cables in the hopes of generating current
> flow through the substrate.
There's a large matter of degree here. I once measured the discharge
through each lift tubes of a UGF at 160 gallons/hour - so a total of 320
gallons/hour circulated through the substrate. The substrate contained
less than 2 gallons of water (3 square feet of substrate, 3 inches thick
with 35% porosity), so it took only about 23 seconds for the ugf to turn
over all the water in the substrate. You might say the the average
"residence time" for water in the substrate was 23 seconds. In fact some
parts of the substrate would probably get little or no flow while other
parts would turn over much faster than that.
This very rapid turnover doesn't allow very many reactions between the
water and the substrate and means that the chemical and biological
environment in the substrate is pretty much the same as it is in the
rest of the tank - your plants (almost) might as well be floating.
Heating cables or "slow" ugf systems circulate the water much more
gradually. I estimate that the slow ugf I'm using in one tank now
provides an average residence time in the substrate of about an hour - I'd
prefer it to be longer than that. I'm not sure how fast heating cables
would turn the water over, but I'll guess that it normally takes more
than an hour.
Slow turnover allows interaction between the water and the substrate; e.g.
it allows oxygen in the water to be depleted by bacterial activity and
allows nutrients in the water to bind to exchange sites in the substrate.
It also allows time for nutrients that are released within the substrate -
ammonium from burrowing snails, for instance - to be used before it
circulates back into the open water in the tank.
Slow circulation provides a way to replenish nutrients in the substrate,
and to promote a diversity of biological activities there. Without it the
substrate will probably gradually get depleted. Then you have to start
plugging in lily spikes, plant tabs, laterite balls, etc, etc.