These dwarf puffers are 100% fresh water, not brackish. Check out www.dwarf puffers.com for more information. As for your question about their tails, I've read that it can be a sign of stress but I think it might just be a general characteristic as well. My puffers tails are almost always not fanning and are almost always curled in which ever direction they are turning. Mine tend fan their tails when they get in little arguments or when they're darting after some food. Sometimes they fan them when they're resting. I can't really get any patterns down, but I think probably about 70% of the time the tails are not fanned. Maybe I'm just a bad poppa with bad water conditions, but I don't think so. Does anyone else have any experience with this?