hmm. can you give more of a description or photo? round black and crab like sounds like mites, but most mites that i see on decomposing matter are white. there are so many types of mites that it is hard to say if this bug is a mite and if so, is it beneficial or harmful? I hear black and red mites are to be stayed away from for all pets.
Sterilize everything! new substrate, bleach ratio with plants. water:bleach 10:1 Rinse rinse rinse after you bleach. there is a chemical commonly used called Betadine.
Here is a link that will give you set by step instructions. Please read through it because the first sign of mites will save you from an infestation that will spread to other tanks
http://www.anapsid.org/mites.html