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Reducing diet

I am not recommending what follows, just reporting interesting data.

About 2 years ago I set up three 10 gallon seeding growout tanks.   
The substrate was 2mm gravel with laterite below.

Since I had tanks, I put fish in them.  In one of the tanks I spawned H
cristatus and raised the fry.  Eventually there were 24 2" cichlids in a 10
gallon (over 1" per gallon I guess).
I was running an Aquaclear mini.  Needless to say the fish made short work
of any plants
and devoured Tetramin like there was no tomorrow.   I was feeding about an
ounce a week.

The water was turning tinted so fast I was changing 30% every day or two.   
Finally in desperation (down to about 15 fish at this point) I tore down
the tank.

Where there had been laterite below the gravel, there was now a mixture of
and COMPLETELY REDUCED IRON.   It looked like tiny iron filings.   

I am pretty sure this black stuff was not a sulphur compound (tho it could
have been FeS) 
because it was very dense.  

That says something about how anaerobic and what strong reducing conditions
can exist
in the gravel.   

Interesting, no?   Comments?  (Other than "don't do that!")

Dave Gomberg, San Francisco            mailto:gomberg at wcf_com
FormMaestro                              <http://www.wcf.com>