Hi- This depends on the size of the tank and if it's a freshwater tank. Let's say it is a ten-gallon fresh water tank. Put fish in separate bowl of new water. Remember water from the tap has chlorine in it, so hopefully you have those dechlorinator drops. Put gravel in another large bowl and rinse it. Then what I do is use baking soda and swish it all around to clean it safely. Rinse thoroughly- when you think the gravel is clean, rinse it again 2x. Use cold water for final rinse. Clean the tank with water and baking soda. Rinse well. Clean all decorative stuff. Refill the tank with fresh cold water- place the gravel and decorations back in it. For a ten gallon tank, put drops of the dechorinator in, according to directions. Voila'! Done.