Some Puntius species, like P. ticto, do occur in slightly brackish water. So if your tank is only barely brackish, SG 1.002 say, then it's quite possible that these fish have become acclimatised. Because of that, don't do anything sudden; do your usual 20-25% water changes each weekend, replacing brackish water with plain vanilla tap water. Or, if they seem happy enough (and they look healthy) you could carry on as you are doing.
Cheers, Neale