i just bought a stocked 135 gallon tank, it had a massive pink leather in it, it didnt handle the move to well and died on the second day, i came home from work and it was a large ball of slime, with yellow stuff exposed and it was just falling apart, so i took it out and checked my water and the ammona was at 1ppm (i expected it to recycle as we took out about half the sand becuase it was so heavy and i almost dropped the one end so i thought we had better take something out to make it lighter) the next day (did a water change after i took the leather out and checked water) the ammona was already back down to .25ppm so i belive it hasnt started the real cycle yet, but if it hadnt started with that leather dieing its about to if its going to (i hope its not to bad sence it has the nicest looking yellow tang ive ever seen in it)
anyways i belive dieing or dead corals can and do change your lvls in your water