For fish from an online store you shouldn’t float them with the bag open for too long or you risk ammonia burns from the shipping water.
My Methods:
Online Fish:
Float bag to temp acclimate, while it is floating cut a small hole and suck enough water to test the salinity with my refractometer, then immediately seal the hole with duct tape. Then while it’s floating I match the salinity of my QT to that of the bag. I normally start my qt a week or so before I get the fish, and start it at 1.020, that’s the salinity fish from my LFS come in at so it’s always a good starting point. Then I raise or lower it with RODI or fresh saltwater at 1.026 until it matches the shipping bag. This whole process takes around 30 minutes or so. Then once the salinity matches I’ll untape the hole in the shipping bag and check the temp inside really quickly. I usually shoot for within a half degree or so, and once temps match I’ll take the bag out of the tank, cut it open and pour the whole bag through a colander, into an empty bucket. then take a few scoops of fresh RODI and rinse off the fish with that while it’s in the colander, then put the fish straight into QT from the colander, this way there is no water transfer from the shipping bag to the tank. After this I take about a week to raise the salinity to display levels, then start any qt meds I feel are necessary based on observation.
You don’t necessarily have to take this long to raise the salinity, it can be done in an hour or so, just HAVE TO HAVE TO HAVE TO get them out of the shipping water very quickly.