The same thing happened to me, and only once. And it sucks and is one of the most stressful things to deal with.
I see that you're in NY, where there's a lot of rain/snow this time of year. Some water treatment facilities add additional amounts of chlorine (which is a gas), or possibly Chloramine (which is a solid) depending on where you live, during high rain situations. If you haven't researched this topic, do it.
If this is true, you have killed most, or all, of your bio load with those 50% water changes. Moreover you continue to kill your bio if you're not purging the chlorine before you add it to the tank. The fact that you ran the tank fine for a year almost confirms my theory. You may have enough bio media now, more never hurts, but it's not going to fix your problem at this point.
First, you need to get a TOTAL<---
Chlorine Test Kit like this and test your tap water. Also, get with the treatment facility and find out if they use chlorine, and then start using a bucket with an air bubbler to remove chlorine before you add it to the system (again, research). If they use Chloramine, an air bubbler will not work and you’ll need to research how to remove it.
Second, get the fish out of there. All of us have a few LFS with which we have good relations, and they may board your fish until the issue is resolved.
DO NOT add ammonia remover. This would be silly since the new nitrifying bacteria need that ammonia to start the new cycle.
The ONLY chemical you should be adding at this point is some sort of "bottled bio". BUT DO NOT ADD bottled bio before you're certain the chlorine is gone, or you’ll simply kill it. After you do add the bio, you basically have to stop analyzing it and walk away for a couple of weeks (painful, I know).
The bottom line is, you have to start the cycle from scratch.
Good luck, and I feel your pain..