you could also use your fingers instead of a blunt knife to just rub the slime off after soaking it in hot water.
If you have a problem using the hot water as a process, try dipping the fish into the hot water and removing it promptly. Or you could use the alum method to wash off the slime. soak alum and fish in some warm water and wash gently with your hands.
You could try the use of ash and a sponge too