Why would they? Its well known that once a fish dies (a big fish - for the tank volume) - if not removed from the tank - that there can be a cascade of death - whether from bacterial overgrowth/low O2 - or ammonia, etc. I don't think many people measure ammonia except with cycling - and I personally am not Sure 'why'.Has anybody actually measured the ammonia level in a cycled aquarium with a dead fish left to rot?