If you read up on MTSes and Ramshorns, they are pretty salt tolerable. "The Malayan livebearing snail positively thrives in brackish water and makes an excellent addition to a tank with a sandy substrate, where it will eat up any leftover food or organic matter and thereby prevent any problems with anaerobic decay. The Colombian Ramshorn snail does well in slightly brackish water, but it is a notorious plant eater."