If any of the conditions are significantly different between the tanks (by "conditions" here, I mean not only things like temp or pH, but also for example things like nitrates...while 80 ppm nitrates is not normally toxic to a fish, I would be worried about netting a fish out of a tank with 5 ppm nitrates and just tossing him into a tank with 80 ppm nitrates, etc.), then what I do is use a small container of whatever sort (a clean tupperware, or anything like that) and catch the fish in the old tank and fill the container about 1/3 or 1/2 with the old water. Then gradually over 20 minutes or half hour or so, keep adding small amounts of the new tank water every 3-4 minutes. Once you have the container 3/4 or so filled up, then just net the fish out and into the new tank, or even just lower the whole small container right into his new tank. (If you are sure both tanks are disease-free, then this is the easiest option and poses no hazards, and is less stressful on the fish since it is never out of water.)