black patch under sand
having just read the advice of the question posed regarding, "black spots on sand"it occured to me that i could have a potential problem and probably need to ask a question and rectify any potential problems.
Approximately a month ago i introduced about 10 australian yabbies, these are a near transparent crustacean, that live in sand around some of our creeks, they are easily obtained with a yabbie pump and i thought they would make good sand filters and maybe eat some of the detrius that accumulates on the sand.
Anyway, i think that a couple of them may have died and i now have one very large circle shaped, black or maybe dark purple stain against my front glass under the sand, it is approx 3" x 4" round.
There is also another smaller circle about three inches from the first one.
I surmised that it was just the bacteria feeding off the dead creatures and that it would go away in due course. This has not happened and my questions are as follows, can it be harmfull, should i dig it all out and remove it and what is it???
thank you all for spending the time to read this and any suggestions you may have..