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Anaerobic areas in tank

Good day everyone,

I have seen several questions posted on anaerobic areas in substrates and 
fears people have of these.  I now think I have some anaerobic areas in my 
old 20 gallon tank and have posted some pictures of this tank so I can 
verify this.  I will leave them up for the benefit of people wondering what 
these areas may look like

I first set up this 20 gallon high aquarium in October of 1997.  It had a 
substrate of subsoil, vermiculite, and micronized iron, which was covered by 
large grained sand.  This tank did well for several years and had swords, 
aponogetons, anubias, even a lace plant.  I then set up a larger tank and 
moved some of the plants out a year ago leaving only java ferns and 
sagittaria species.  Lately, growth had slowed and I decided to add clay 
balls to the substrate.  I took some of the original soil I have kept in a 
plastic container and rolled this with micronized iron.  I made about 12 
balls and inserted them into the sand.  After a few weeks black areas began 
to show up.  I've never had anything like this happen so I decided to 

I dug up some of the black areas.  What I found was that these were 
concentrated where I placed the clay balls.  The sand had turned black and I 
don't think it was from the soil since I washed the sand in water and the 
grains remained this black color.  The really unusual thing is that after 
two days of leaving the black grains on a paper plate they returned to the 
normal sand color.  As I removed this black sand from each area, a stream of 
bubbles would come to the surface.  I believe this is a clear sign but there 
was no smell to these bubbles.  I smelled the black sand and didn't smell 
anything strong.

Questions I have:

1. I may have added more micronized iron in the clay balls than before.  
Would this contribute to the problem?
2. Should I gravel vacuum out all of the clay balls? Will this eliminate the 
3. Once you have these anaerobic areas will they continue to spread?
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 
5. Why did the black color on the sand fade away after a few days?  Is this 
some type of chemical reaction?

Please check out my webpage at:


I am hoping to verify that these are anaerobic areas based on my 
observations and thoughts of others and find out what I can do at this 

Here is a description of the pictures on my site:

Picture 1 is a circular area in front of the tank.
Picture  2 is the side of the tank with a much wider and darker area.
Picture 3 shows the black sand after being removed from the tank.
Picture 4 shows the whole tank.

I have two other tanks and my 90 gallon, which is my main tank.  I'm 
probably going to move some of the java ferns out and take the tank down.  I 
set this website up out of curiosity since I have never experienced this 
before and hope to find out why this happened.  I have flourite in my other 

I would be interested in the chemistry of what is going on here also.



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