Slow and steady wins the race... sturdy cotton pillow case with lots of tiny holes in it for draining works great for fish w/out barbs ect to get caught. drop the bag in the tank, let it chill with the fish until the fish swims around w/out issue with it. then slowy/gentaly herd the fish into the bag. close it, wrap the fish up firmly but gentaly. and use it to xfer into a cooler. get the fish out of the pillow case and xfer both to new spot. wrap fish back up and xfer to new tank.
Big fish and thrashing/raceing around the tank is a good way to do some seriouse damage to the fish and potentially yourself. Don't ever be "pressed for time" when moveing a fish, and have patience.
btw use this method all the time at work with large plecos take a large cotton cloth, put it over the fish. hold it's pectoral fins against it's body and it's top-fin firmly down ( never forceing the fins to lay flat against the body, when the fish relaxes they can be held down easily, alot like a crocs mouth).. and lift the fish out and into a bucket/cooler ect. Fish barely thrashes once it's caught, firm but not tight grip is key. it's eyes are covered by the cloth and it stays damp/no sudden temp exposure/change.
