I'll back masone on the fact that the critters breed in saltwater but can grow to quite large sizes in total freshwater. In fact, several Queensland impoundments have been stocked with juvenile and brood stock mangrove jack up to 5-6kg (bit over 2 feet in length).
However, often, this winds up being the issue:
amehel0;3412841; said:
i got one! awsoem lil fellar and teething. there alot weaker in freshwater. i recently moved mine and he developed fin damage it wasnt healign so i put him in brackish and he healed ver quickly now hes back to full fresh. he eats fish strips (ive got 6kg of fish fllets).
The solution is generally to house them in fairly hard, alkaline water. They do better in that than they will in the soft stuff. Reason being, while their physiology can handle being in freshwater, it's much better suited to the high mineral content of saltwater.
While I'm going to grossly over-simplify this, the reasons why most saltwater fish can't handle freshwater, and vice versa is due to the laws of diffusion/osmosis.
Osmosis: If a cell has a lower mineral content than the surrounding solution, water will leave the cell until the ratio of 'mineral to water' within the cell matches the outside environment and vice versa.
Diffusion: Same principle, but with minerals. If a cell has a higher mineral content than the surrounding environment, then the minerals will diffuse across the cellular membrane until the concentration on either side of the membrane is even.
Basically: If a freshwater fish was dumped into saltwater, what generally happens, is that the fish is unable to stop water from escaping its cells - the cells collapse, and the fish dehydrates. Conversely, if a saltwater fish winds up in complete freshwater, and it can't control the mineral/water balance in its cells quickly enough, water engorges the fish's cells and eventually bursts them. You won't find a shrivelled, or exploded fish in your tank, but that's what's happening on a microscopic level - not a nice way to go.
Some fish are capable of regulating how severely osmosis and diffusion affect them by retaining/excreting salts or water etc. while undergoing physiological changes (especially to their kidneys) to make maintaining their ideal body chemistry easier.
Very few, if any fish are capable of switching from a hard, saltwater environment to an extremely soft, freshwater environment in a short term. Physiological changes take time.
Soft water is called this because of its lack of mineral content in the water - water that's too soft will still result in tissue damage in the jack due to the laws of osmosis and diffusion. There's only so much a fish can do.
So, if you were going to keep a mangrove jack in freshwater (and it was recently acclimated from saltwater), I'd reccomend water with a high mineral load. You might want to try this anyway if you wind up with one that's looking a little bit sickly and mottled (generally, that's how they express stress).
he lived with a pleco the same size and never bothered him if they grow up together the generally are ok. 
