Based on the picture your tank readings are NO3 0, nitrite 0, GH75, Cl 0, KH 0, PH 7.2 (all readings PPM). Note camera can shift the colors a little but I don't think that is significant. Also note the tetra stripe in the picture does not check for ammonia. you should purchase a liquid ammonia test kit.
What was the copper reading and what was the ammonia reading? You don't have to post a picture of the test The numbers will work. Also could you test your untreated tap water with a strip and post the numbers for that.
I know you said everything was OK but to be honest we have head that before only to find later the actual reading was high You don't have to post a picture of the test The numbers will work.
I use tap water with a dechlorinator. I’ve only had to top off once.
No idea where to get a water quality report from.
Go to google.com and type in "Water Quality report" followed by the city / county and name of the water utility. In the US most cities post this on the web. IF it is available it might provide a lot more data. Whenever possible use the purist water you can get for water top offs.
I haven’t done a water change yet since the fish were only added about a week ago
Regardless of when you added fish, plants, snails and shrimp you should do a water change once a week. IF the fish were added a week ago when were the snails added ? Were water changes done once a week when the sails were in the tank?
Substrate fluval plant/shrimp stratum
Some crushed coral mixed in
I have seachem purigen and polyfilter I just recently added after the deaths. It was just 4 carbon filters in the back.
The hornwort all died and the sword isn’t doing well. The water sprite in the back seems to be starting to brown.
I added flourish tabs recently to the substrate to try to help with nutrients about the same time I added the polyfilter
The coral will stabilize PH and the substrate probably has some mineral nutrients but those will be removed over time due to plant growth and water changes. Root tabs will help. However I don't see any fertilizer for the water column. For best results you should dose the water with fertilizer.
I am concerned that your plants have stripped the water of all nutrients it had and if you were not doing weekly water changes the plant would have quickly run out of nutrients. Also your plants may have removed all nitrogen (nitrate, nitrite, ammonia and organic nitrogen from the water. The lack of a water columb fertilizer, plants dying and your low nitrogen readings would support this.
I would recommend getting a good liquid fertilizer. I would recommend Thrive (available a nilocg.com). Dose according to instructions on the bottle.
As to the snail deaths at this point the only thought I had was that the didn't have enough beneficial bacteria present to control ammonia. The lack of nutrients in the water may have also affected them. The addition of the snails my have triggered an ammonia spike that you didn't notice and then the plants may have consumed the ammonia before you tested the water for ammonia.
This is a new tank with fish in it so for now I would get the fertilizer and keep a close eye on the PH, GH, nitrate and ammonia. I would also start adding a liquid fertilizer to your water and purchase a liquid ammonia test kit. and do once weekly water changes (50%). and update this thread with your progress.