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Re: Anaerobic areas in tank

Kurt Sloan described the appearance of black areas in his substrate after
he placed some clay balls laced with micronized iron.

My understanding is that the black areas associated with anaerobic
conditions are usually caused by precipitation of iron sulfide on the
surface of the sand grains.  Iron sulfide is a strong coloring agent and
it takes very little of it to blacken the sand.

Correct me if I'm wrong, but isn't micronized iron just very fine iron
filings?  If it does contain metallic iron then possibly what happened in
your tank is something that happens sometimes in iron pipes.  The iron is
a strong but usually not very reactive reducing agent.  Some types of
bacteria can utilize metallic iron and sulfate in the water to produce
ferrous iron in solution and reduce the sulfate to sulfide or to sulfur.
The bacteria live on the energy released in the reaction.  The ferrous
iron and sulfide combine to form iron sulfide.  I think this also produces
high pH conditions, so the released sulfide remains as the HS- ion (which
you can't smell) rather than forming H2S, which is a gas, not very
soluble in water, and very stinky.  The bubbles you observed were probably
odorless nitrogen bubbles formed from denitrification in the substrate. 

> Questions I have:
> 1. I may have added more micronized iron in the clay balls than before.  
> Would this contribute to the problem?

Yes.  I'm not really sure why you didn't see this problem at the outset.  
Perhaps when the substrate was new it couldn't support bacteria that were
able to attack the iron.

> 2. Should I gravel vacuum out all of the clay balls? Will this eliminate the 
> problem?

Possibly.  But as Tom Barr pointed out you probably don't need to worry
about it as long as your plants remain healthy.  In my experience, plants
do just fine with anaerobic zones in the substrate.  In your case the
plants may benefit from dissolved iron released in the anaerobic zones.

> 3. Once you have these anaerobic areas will they continue to spread?

If my conjecture on the mechanism is correct, then they can continue to
spread until the iron source is depleted.

> 4. As you can see from the pictures I have quite a bit of MT snails.  I'm 
> thinking they may have contributed to the top of the substrate clogging up 
> somewhat.

Normally I expect that burrowing MTSs will keep the substrate loose and
help prevent the substrate from "clogging".

> 5. Why did the black color on the sand fade away after a few days?  Is this 
> some type of chemical reaction?

Iron sulfide can oxidize spontaneously in moist air to produce ferric
oxyhydroxides and sulfur dioxide gas.  The black color disappears when
that happens.  The ferric oxyhydroxides produced in the reaction are
brown, reddish or orange. In your picture it looks like your normal sand
is brown colored, so a very thin added coat of brown iron compounds may be

Roger Miller