thick mud is not a problem, but if the thick mud is harboring anaerobic bacteria.
IF the mud smells like sulfur or rotten eggs, then the mud has hydrogen sulfide gas,
and that is NOT GOOD for fish. it will tox their gills. also, when stirred up the
bacteria will suck up all the oxygen.

it is best to remove the muck using thick pads of polyester batting without stirring
up the muck, letting the fish rooting around gradually stir it up. OR, move the
fish, clean out the muck, refill the tank and put the fish back in. Ingrid