The first step is to evacuate the fish safely. Disconnect all electrical and heating supplies and connecting wires to the tank. A fish bowl is the best thing to use at this stage. If you do not have a fish bowl, any clean glass container which can hold the fish and enough water will be appropriate.
Use a net to catch the fish from the tank and drop them into the temporary home. Handle the fish as gently as possible. You can also use the water from the existing water tank for the fish so that there is no change of medium.
Remove the ornaments that you have kept inside the tank. This include the shells, gravel, artificial plant and other décor. Place this aside. Wash them in clean water and rinse under running tap. Place on a tray and let to stand to air dry. A siphon tool can be used to clean the gravel.
Scrape the interior of the fish tank with a pad to get rid of the algae layer. Use a vacuum attachment to pick up the gravel debris at the bottom of the tank. Salt can be used as a scouring agent to get rid of any residues on the walls of the tank.
Soap should never be used to clean the tank because no matter how much it is rinsed, there is some residue left behind which is toxic for the fish. Clean the filter material thoroughly. It can also be replaced conveniently if it is too dirty and rinsing does not help.
After cleaning the tank, remove as much as thirty percent of the water in it. Replace it with clean tap water. Since tap water has added chemicals in it, get chlorine remover from the market and add it to the tank to make the water suitable for the fish.
Replace the gravel and other décor items inside the tank. Plug back the electricity and heating. Run the filter for an hour. Now put back the fish back into the tank.