Hey there.
Our tank here (158g) once held around 30 barramundi, as they got bigger, we sold them back to the pet store and kept about 6- but they were successfully converted from brackish to fresh when we first got them. Have also done the same with all 3 of our Mangrove Jacks, so I'm guessing the technique for converting them works.
The trick is (with chosen fish in separate tank/ big plastic storage container) over 3 or more days, take one third of the water they are in and exchange it for fresh. Then the next day (24hrs later) remove one third again and replace with fresh- and so on until the water is basically all fresh.
Just keep an eye on the fish as you go, and if he looks a little off, leave him for an extra day to adjust before replacing the next lot of water. Feed as normal and obviously don't forget to provide some air/ filter.
If the fish was already living in a tank, just leave (approx) one third of the water that is already in there, and replace the rest with fresh. Do this on the first day so the water has time to run through the filters and recirculate the good bacteria.
(You could probably do this entire process leaving the fish in the tank- but I like to give everything a good clean while the fish is out of the tank to get majority of the salt out of everything and rearrange. I try not to stuff around with the filter bits too much though because they keep a lot of the good bacteria etc)
All in all though, Jacks and Barras are pretty tough and can handle a bit more than most fish. If you like, spread the process out over more days so the fish has more time to adjust to the change of water.
Let me know how you go!
(Tip on this technique was given to us by our amazing and extremely knowledgable local fish store guy.)
-I'm not a professional when it comes to giving advice on anything fish or aquarium related, just so I've covered my ass. Lol
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