All it takes is one fish to die to crash your tank.
That depends highly on the tank. If there isn't enough biofiltration and the system as a whole has a small volume of water, then yes, a single decaying fish with no cleanup crew to dispose of it, could cause a large ammonia spike. However, in this case, a 55 tank that's been running for 6 months with LR
and (hmm no mention of substrate, but I would assume some? what substrate did you have Randy?) There should have been enough biofiltration to handle a dead fish Even two if they were only in the tank for a day or two.
Copper is really only toxic to the invertibrates in your tank. At worst, the pennies, if they had been in there for a long time, may have lead to the death of the snails and crabs, but very doubtful they had anything to do with the fish deaths.
Were there any power outages while you were out? Everything may have been up to temp and filters running when you returned, but if things were off for a day or two while gone, that could have led
to some problems.