marshmallow ….. (sorry, not much help.) Ideally by making sure his tank is in the right healthy parameters, the best and safest way would be a drip acclimation. pretty much slowly dripping water over a decent length of time into another container with your fish so it doesn’t go into a shock. that’s the best way from what I know… I’m sure others could help more in depth. just be sure the temps are the same. if your new tank is heated as it should be, have your other fish bob around in the new tank (while being in a cup or something) to be the same temp.
I’d say take a lot of water out of the tank he’s in, or get him in a cup would be easier, then slowly put some of new tank water in with his old water. Basically drip acclimation. So he can get used to the new parameters and temperature. Once it’s mostly new tank water, gently let him free in the new tank.