yeah most live-bearers are brackish water fish (guppies, mollies, platys, etc...) and mollies do the best in a true marine environment (of all the live bearers). there are puffers (yellow-sot) that are brackish, and fish like mono-argentenus that are brackish also. there are several fish in the fish store that do well in salt or fresh and vice versa. i think a brackish water tank would be fun.