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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
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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