Substrates are ticking time bombs that have killed almost as many tanks as faulty heaters. So much crap gets built up and locked in there especially hydrogen sulphide gas which gets released when the sandbed is disturbed.
Best thing is to just toss out the substrate on a tank move or hit the reset button using bleach to kill everything and start all over again with pristine "new" sand. The live rock holds more than enough goodies to restart a new tank.
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