Heat coils: too much heat?

> Joanne wrote:
> I am also wondering if just substrate circulation by UGF is as good as
> heating cables since my 125 gal tank looks just as good as a friends 110
> gal tank with Dupla setup [snip]

I'm beginning to wonder if the slow circulation inherently provided by
convection through heat coil usage (allowing time for ion transfer/
substrate re-charging) may be the biggest advantage of heat coils
(although I see how the plants may want their feet warm also :-).
However, could this slow convection not also be achieved through greater 
attention to slow UGF filtration?  Maybe bubble-lift tubes have a 
purpose after all?  

> My current worry is that undergravel heating cables may overheat the tank
> too much. [snip]

Yes, I have this concern also.  Heat coils work on a relative principle:
a convection is produced only if the coils raise the temperature of the
water around the coils to some threshold *relative* to the surrounding
aqueous temperature.  If your tank is already 90F, the coils must heat
the area to 90+F to generate a convection.  

I do like the concept that the coils could heat the tank from the
substrate up, and I like the gentle convection that may re-charge the
cations in the substrate.  However, if the heating system is integrally
tied to the substrate's bacterial activity, you can't turn the heat
off when the tank gets too hot, lest you risk anaerobic activity in the
substrate.  This could pose a problem for tanks in warmer 
climates/facilities, or in aquariums dedicated to cold water fish.

David Webb (dwebb at ti_com) sent me some of his plans for a substrate
circulation system that I thought looked pretty good (they went through
this mailing list a while back).  Among other things, he had a PVC
lattice in the substrate with a tube extending up the aquarium to above
water level.  With this design, the system is gravity-fed because 
the water level above the tank level represents the hydraulic head that
forces substrate circulation (and measures pressure in the substrate), 
and you could also measure how clogged your substrate may be (the rate 
of head drop when a known quantity of water is added to the substrate).  
Further, you can add your Dupla drops directly to this vertical tube 
and have your nutrients go directly into the substrate.  I liked this 
best of all.

I had thought that it would also be a good way to do CO2 injection
right into the substrate, but I now think that if I keep the flow-rate
low (like I think it should be), I won't be able to get enough PPM CO2
injected into the substrate.  HOWEVER, if I could rig a way to have
my reactor *IN* the substrate, I could...?  That would certainly 
provide the greatest efficiency for leaving CO2 in the aquarium for 
the longest possible time, but would be very slow to adjust the
system's pH and be difficult monitor the rate of CO2 dissolved.  I
suppose I could put a pH probe in the substrate circulation system
and calculate the rate of change to the tank (with two known pH's,
but a calculable rate of flow from the substrate to the rest of the
tank).  I suppose I could also put the reaction chamber up against the
glass inside the substrate so I could see the bubbles.  I will have to
give this some more thought.

cbay at lookout_ecte.uswc.uswest.com