I am pretty sure that stands for "Freshwater", but you can confirm with liveaquaria if you have a question. Pea Puffers come in two varieties--brackish and freshwater. The Freshwater variety have been bred and raised in freshwater and are accustomed to that. The brackish variety will tolerate freshwater up to a point, but as they age will need brackish in order to thrive.In regards to liveaquaria, all there listings have two purchase options on each fish. One has a Freshwater noted beside it and the other does not. Anyone know what the Freshwater stands for? No difference in price with or without. Just curious.