Re: diffusion

Dave Gomberg wrote:
> The book is Limnology by Tarkington(?) and it is the classic work
> in the field.  His point was that disturbance was the name of the game.  
> If you depend on diffusion you will never get there.  

Where? ;-)

Go figure. Diffusion: "the process by which liquids, gases or solids
intermingle as the result of their spontaneous movement caused by
thermal agitation and in dissolved substances move from a region of 
higher to one of lower concentration." The speed of this process is
highly dependent upon temperature. It will also be dependent upon
the molecular size and the chemical affinities of the molecules or 
ion species. I expect the ionic charges are very important too. From
that we could conclude that Fe-EDTA diffuses through substrate much
slower than NH4+. Slow substrate circulation is good; everybody agrees.

Lakes in temperate regions undergo a thermal inversion twice a year
when the hot and cold water regions flip/flop. This creates a lot of
mixing. Maybe Tarkington was making some reference to this. Lakes
are never completely undisturbed. Was Tarkington making reference to
a specific study of oxygen concentration migration?

Anyway, I surrender. This is a minor point. Malasian snails, or some
other method of injecting water under the substrate once in a while
would undoubtedly greatly assist getting nutrients in there. I suppose
we could mimic terrestrial gardeners and poke it with sharp implements
periodically. ;-)

I recall that Jim Kelly put pond fertilizer tablets into his
vermiculite substrate and I suppose this is another way to get
nutrients into it especially if we're pretty certain now that once
in there, they are pretty much trapped there; prisoners of s-l-o-w 
diffusion! ;-)

NEW Subject: Is 500w MH lighting too much in an 18" deep x 4' tank?
It turns out that 175w and 250w lamps are considerably cheaper than
other wattages due to their common use. Is there an upper limit
upon the amount of useful light you can pump into a plant aquarium?