This is from Lion Crazz 101 tips thread....I'm too tired to type it myself
23. ALWAYS PREMIX WATER BEFORE ADDING IT TO YOUR TANK. You never want to just mix water with salt right on the spot and just add it to your tank (except for just setting it up – since nothing is in the tank). You need to premix it because if you do not, you may be drastically changing the water levels and making the water very unstable. Always premix the water and make sure the salt level and temperature is exactly where it should be before adding it to your tank.