Everyone died

Okay. I posted earlier today asking for some help, saying I was at my wits end, now I’m really at my wits end. I had gotten a pleco from my LFS. I went on a 4 day vacation. Came home- several fish had ich. I had 3 causalities during this ich treatment. I ended up doing a 50% water change. I was only supposed to do a 25%, but for reasons I do not feel like explaining, accidentally ended up having to do a 50% water change. After this, I went to my LFS again, to purchase some aquarium salt for my silver dollar that was having some cloudy eye issues. Came home and to my surprise, all but 4 of my fish died. I’m talking 15 causalities. Not including the 3 that had already passed. I am at a loss of words. Checked my water parameters after this and my ammonia levels were at ~1 ppm. The 4 survivors are in a quarantine tank and they are seemingly fine. For now at least. During the ick treatment I had turned my filter off, as the directions said to remove activated carbon. I’m thinking I ruined my cycle. I just removed 50% of the water in the tank, and I’m not sure if the sudden deaths were due to the water parameters or some type of something in the water. I’m at a loss of words and I’m not sure what to do.

This was a well established, 75 gallon tank. It has never given me any issues. I don’t do frequent water changes, as my parameters are usually fine. After the massacre this water change brought, I decided to test my tap water as well. I will attach pictures. Both the tank water and the tap water have 0 nitrites. Prior to this whole debacle (the ich, the treatment, and the disaster that ensued) my nitrates were around 40 ppm. Not sure why that spiked either.

Also, somehow my tap water has more ammonia than my tank water. Not sure how that is possible.

What do I do? I’m already going to have to go ahead with the second 50% water change of the day, but should I completely drain the tank? I’m not sure if there’s some chemicals in there that caused all my fish to die within a matter of 3 hours.

Someone please help. I don’t want my last 4 fish to die.