CichlidFish.com - Fish That Think.

I saw the cutest little puffer fish in the lfs the other day. They said he was a yellow puffer.

The tank said brackish water fish and when I asked what it meant they said it meant a mix of salt and fresh water.

I had heard that brackish fish are fish that can go in saltwater OR freshwater. I really wanted to buy the little fellow but I did not want to kill him.

I searched brackish water fish when I got home and came up with the same conclusions. Salt OR freshwater. I looked at the list of fish that are and they said Mollies are brackish water fish. I kept them for a long time with no problems in freshwater.

Can someone clear this up for me please!?

Mollies can live in fresh, brackish, or full maine conditions.  Certain other fish live in one or another at different stages in their lives.  Most brackish water fish venture comfortably into fresh or salt, but their optimal living conditions are "brackish".  Has to do with how their systems are set up to regulate the balance of salts content within the body.  Freshwater fish need to keep their bodily salt content from osmosing into the less salts-laden fresh water they live in.  Saltwater fish need to keep the greater salt content of seawater from overwhelming them.  So it's the degree of regulation that these fish have evolved to handle that is the qualifier.  Don't know enough about puffers to respond to your particular fish, but that's the generality.