Do you have or are planning a separate tank for them just in case? I only ask because plenty of brackish fish may do well in freshwater for a period of time and then suddenly turn for the worse. Since they're such a manageable size a ten I imagine would be fine. I'd also inquire the specifics of the water conditions they're currently being kept in as brackish fish like hard water and some of their being okay in the freshwater they're currently kept in maybe due to the water conditions of their tap since some areas have higher levels of calcium, etc. Do they know the catch location by chance?
I'm sorry if I'm all over the place just going through the motions I went through when I was considering picking up some Colomesus psittacus from Wes. I hate losing fish and tend to over think things sometimes.