The pH literally could not go to zero or negative because the acid you are using is more basic than that. Realistically the farthest you could drop it is around 2 or 3, which is similar to vinegar. So in reality it would not be harmful to human skin, but would make eyes burn like crazy. It would begin damaging the liner, pipes, and pump, but the issues would be due to chronic exposure and acute exposure wouldn't cause any significant damage. The exception would be a heater, which has copper tubing with a large surface area, which would start to see damage almost instantly and could be destroyed within days or even hours.
So the issue is that PoolMath, or any pool calculator for that matter, cannot reasonably calculate large pH changes. So when you told it you were adding 5 gallons of MA it gave you an unreasonable result. Not to say you should add 5 gallons of MA ever, but it would not take the pH down to zero.
But in the tradition of Randall Munroe, "What if?" If your pH were zero or lower any exposed metal would begin getting eaten. The liner would quickly begin to wrinkle and most likely the seals of the pump would start to disintegrate causing leaks. Leaks on to concrete would begin etching it. As the liner failed and the aluminum of your pool began dissolving a catastrophic failure would result. The tsunami of acid would destroy all plant life it met as it swept through the yard. A person in the pool before this happened would quickly be unable to breathe, their skin would burn, they would be blinded by the fumes. As they attempted to escape the acid pool the ladder would fail under their weight, plunging them in to the acid. Their lifeless body would be deposited somewhere after the tsunami for the investigators to ponder. It would be a bad day all around ?
So, while such a fate would not be in store for you or the pool, please continue to advise people never to make such stupidly large adjustments to their pool. Lot of bad advice out there, unfortunately...
