I think 13 fish in a 72g, especially with some of the fish he's keeping, the duncans and other LPS would also contribute to the bioload if they are being target fed, there would be the source of the nitrate. Combine that with a new tank. Factor in a not 0 ammonia reading.
Ding Ding Ding. This is one of your problems.
Feed your tank as you normally would, then test your ammonia in an hour, then in 4 hours, then in 8. If I had to guess your fish and corals are being poisoned by to great of a bioload and a bacateria population that is not significant enough to clear that load.
City water,
Are you treating the water with anything. Doing that amount of water changes with untreated water is going to cause you a problem as well. It's not the TDS or phosphate, it is the chloramine and other elements they add to our water. It also varies throughout the year as to what and how much they add. If your using tap water you really need to be treating it with a filter or a product similiar to Amquel+. Compared to what you have in stock, a water filter would have been cheap. Not trying to rub it in at all, just saying it's worth it.
Maintenance,
I am sure you have heard this, but how often are you checking your equipment. I know your tank is on the new side but how often are you cleaning and inspecting equipment?
I myself do not have a ground probe or any type of meter for it. Cut your hand, stick it in the tank (wait you have triggers maybe not a good idea) if it tingles you got a leach.
HTH
Don't give up bro. This is a little bump. It happens, your still pretty new in your reef career, your gonna have some bumps. We'll help you through them. Just gotta have some faith, some patience and your persistance.