Some snails are immune. Excel killed the mini-
ramshorns
They can grow to up to four centimeters in size. They generally will not become a pest, although they can consume large amounts of plant matter.
› wiki › Ramshorn_snail
Is flourish trace safe for snails?
You'll be totally fine with Snails and trace.
Is flourish Excel shrimp safe?
Registered. It's not exactly harmful to shrimp if you are not dosing too much. As far as I know, there is no copper in Excel. Copper, as you might have already known is really toxic to shrimps...