You could send a message to tricker.com. They sell the trap doors and reply to messages.
per the net
7) Are there drawbacks to using salt?
-It will harm many plants commonly used in water gardens.
-It's inadvisable to use salt in combination with Formalin. Formalin mixed water containing more than .1% salt can be harmful to koi and goldfish.
-In pond situations be aware that salt may kill algae, causing a drop in dissolved oxygen from decay of the algae. Be sure circulation is adequate.
-Trapdoor snails cannot tolerate salt.
per a different site they can handle some salt
itle: Re: Trapdoor Snails - Can they take some salt?
Post by: koi keeper on June 20, 2009, 08:11:39 AM They can tolerate salt levels of .03% easily, thats all the salt you'll ever need.