Can i keep monos in freshwater?

At a fish store, I recently saw that they had two tanks with mono argentus. One housed saltwater fish aswell as monos and s (in full marine)...and the other had monos with plecos (in freshwater). The seller said monos are fine if kept in fresh, brackish, or salt, as long as you aclimate them properly thats why he sold them in different waters. Has anyone had any real experience with monos in FW??