Aquariums: Fish all dying (WARNING DEAD FISH!!!)

The only present symptoms I have seen through my fish so far is that one of their fins will stop moving and like clamp, they’ll become weak and start laying against the bottom of the tank within hours, and then die.

I got 6 guppies Wednesday from petco. They all looked healthy, and I acclimated them to my tank. One died overnight, but I didn’t think anything of it. Thursday, everyone (5 guppies and my betta) was alive when I left. 2 of the guppies had the one fin not moving. Both dead when I got home 6 hours later.
I woke up this morning, and my betta looked like crap. I asked around on another sub and I didn’t get an answer, so I went out for copper treatment as I was told when I asked about the guppies on a guppy sub before my betta got visibly sick.

I came home 1/2 hour ago, and he was dead. Same symptoms, one fin not moving, weak, sitting at the bottom of the tank.

I have the remaining 3 guppies I have in a separate tank now and I’m giving them copper treatment, and I’m hoping they survive. But I just want to figure out what’s going on. (Separate tank from my main tank because I have snails in my main tank)
I’ve had this tank for close to 5 months now. Had my betta for about the same time (a Christmas present) got the snails in late January, and they are just fine.

The last picture is my water parameters. Everything looks alright physically to me, other than the GH and PH are a little low (though ph never got under a safe level, and the employee I talked to said the GH was nothing to worry about) I added some stuff for ph yesterday (I think it’s crushed shells or sum? The employee I talked to picket it out for me.