Re:Yet another plant tank design

>>> Kevin, good design.  I'm doing this now.  I am very happy with it.  I
>>> would do the same thing again.

>>The two designs differ a bit in that my system (which, in fact, was
>>derived from your system, but I couldn't remember who the original
>>designer was) keeps the water and nutrients in the open space below
>>the substrate circulating and aerated.  [snip].

Ok--I think I see what you want.  Circulated and aerated nutrient-rich
water in the space below the substrate.  I can do the same thing by
putting an inline pump between my two tubes going under the substrate,
but didn't because I'm not yet convinced I need such active circulation.
(It would take all of five minutes to set up.) Aeration and nutrient 
additives, of course, were main goals of my setup.

Do you think yours would require an air-tight system, because it goes
above the tank?  I am interested in how you get around this (an
air-tight system would be a pain to deal with, especially when adding
nutrients).  :^>

>Allen Sandifer then asked:
>I have been reading this thread with great interst. I have had the idea of
>placing eggcrate or something similar in the bottom of the tank and placing
>a couple of the 7.5w heaters under it (not resting on the glass). I have
>thought this would possibly work better than a UGH not to emntion the fact
>it would eliminate the unsitely lift tubes.

>My thought was that by heating the water under the substrate it would rise
>through the substrate drawing cooler water down into the lower water
>chamber. In theory it should work much like heating cables.

>Does anyone have any comments about this concept, good or bad and has anyone
>tried it yet?

I thought about this.  I almost did it, but decided against it because:

(1) The heaters are inaccessible.  I would have to rip out the substrate
    to adjust their temperature, or to replace a broken heater.  I could
    control the temperature with some kind of timer or thermocouple
    attached to a switch to turn the heaters on or off, but that is a
    pain (and a lot of infrastructure/expense).  Also, I would still have
    the cords comming up out of the substrate, which I thought was less
    than ideal.  I can't fish them through the PVC drilled through the
    bottom of the tank, because the plug on the end would require a hole
    too large to seal.  

(2) Circulation might be too localized.  I'm guessing the heaters will
    get a good convection going right above them, but will not heat the
    substrate very much laterally.  We (probably) have strong convection,
    maybe a little conduction, but probably almost no radiation.  The
    substrate will probably be warm only above the heaters, although
    the conduction it promotes should circulate quite a bit of water
    through the substrate (down) off to the sides (that substrate will 
    just remain much cooler:  I'm guessing 10-20+ degrees cooler, depending
    on the size of the tank).  If you add a supplementary form of
    circulation, that might help more uniformly heat the substrate while
    still having the strong convection you mentioned.  I almost put
    two heaters down there with one powerhead for circulation, and the
    airline tube into the powerhead would "pull in" the nutrients
    directly to the substrate with full-time active circulation.
    I decided against it, because the plenum would have to get taller
    and because of the accessibility issues in (1).

I ended up thinking that having an external heat source was adequate, more
controllable, more flexible, and less maintenance.  Similarly, an external
circulation source (pump) was more controlable, more flexible, and less
maintenance.  For both, an external source is certainly more scalable:
Just put in a bigger heater to the substrate sump, or a bigger (or smaller)
pump.  I can change my mind later.  

Since I can inject or circulate water directly into my substrate, I can 
add hot water directly and uniformly with a heater set on "high" in my 
"hot substrate sump".  Right now, I'm adding "hot peat-filtered water" to 
my substrate.

I'm thinking I can have a simple power-head connect between two airline
tubes for circulation.  Anybody have any comments on this?

--charley                           Fort Collins, Colorado USA
charleyb at gr_hp.com	or	charley at agrostis_nrel.colostate.edu