Long email; sorry....
Once more test; can you check for copper?
If the tank is healthy, which it sounds like your should have been ammonia should be converted very quickly and you would have Nitrites, and Nitrates. The fact that you don't is not a good sign.
Example:
I once helped a friend diagnose random fish deaths and we found (after a long time) that the cleaning people where spraying ammonia cleaner on the tank glass but by the time the water was tested the ammonia had been converted but the spike had done the damage.
Back to reality...
The facts that ammonia was still in the tank, and if the mag is an accurate reading I would be suspect of the water change.
Can I assume that you have some amount of (real) rock and sand in this tank and it is not bare bottom and fake rocks?
While an 80% water change is on the high side I would not expect it to cause the tank to re-cycle as suggested above. Most of the bacteria lives within the rock and sand and not the water. Such a large water change can stress animals if done too quickly and could cause deaths if the PH and temp (especially) do not closely match the existing tank conditions but a re-cycle; my experiences do not support that; I have done large changes without loosing my bacteria base.
What next:
I don't know what to tell you now, after the fact but I would, if possible, dump all water in the tank and start with fresh salt water. Make some buckets of fresh salt and move your rock into the buckets, maybe rinse the rock in 1 bucket and store into another, if the sand isn't too deep you might be able to rinse and keep it the same way. Any remaining live critters, acclimate into the new water and drain and clean the tank. Think of the bucket as being a temporary tank (heat/cool them as you did the main tank). Fill with new RO/DI (preferably), move sand and some rock (but not all rock because it will help the health of the buckets). Cycle up the tank, which should happen quickly, maybe you can get a piece of rock or some sand from an established tank; that will help cycle faster also. My guess is that within 1-2 weeks you will have your happy tank back.
Yes, this is a bit drastic but I am suggesting it for long term health; you don't know what was in the change water so removing it all eliminates a chance that there is something left over and the tank becomes a tank-of-death for anything you add in the future.
If you happen to measure copper then your sand has to go and maybe the rock; it's very difficult to get the copper out but it can be done but let's cross that if we have to.
In the future:
Test the water this company sends and drip into the tank slowly. One of the best ways to change such a large amount of water is to place your water makeup pump into the new water and use a drip acclimation hose to drain the tank. This way the new water goes is very slowly and doesn't change the tank parameters too quickly. It also gives you some warning if something is going bad and even a chance to stop the change before it kills anything.
