Your water tests seem fine, and snails do act as a “canary in the coal mine” for many issues.24 hours later and the other fish still seems to be behaving normally. He ate normally this evening and I see no signs of heavy breathing or difficulty moving. I've left an air stone in simply because I don't think it could hurt and I'm still worried. I also added a new trochus snail yesterday (I bought it before I returned home and saw the issues) and as of the moment he's also moving about normally. I know snails are extra sensitive to many water chemistry issues so I see this as a good sign.
I don't see anything in my water test results that would have me panicked so not sure what else to do besides wait and see and do my normal maintenance.
Test Results
Salinity: 1.022....need to raise this but not low enough I would think to kill a fish.
Ammonia: 0/"safe" on Seachem alert badge. .25 with API master test kit (best guess, I never can tell for sure with the color).
Nitrate: 0
Ph = 8.0
Phosphate: .04 ppm with Hannah ULR checker
Jay