These sound like isopods.  Isopods are an order of Crustacea, mostly
marine, but there are quite a few fresh water species, as well as
terrestrial forms (the "sow bugs" or "pill bugs" or "wood lice" found on
damp earth under fallen logs, etc.).  Take a look at this page and see
if this is similar to what you have . . .

http://www.museum.vic.gov.au/crust/pillbugs.html

Regards,
Paul M.