Remove the lighting and decorations
First of all, remove any lighting from the top of the tank so that you have unobstructed access. Then remove any artificial decorations or plants that you have placed in the tank after wearing gloves. These decorations and plants should be washed in a bucket of water or under running tap water.
Switch off the electrical devices
Be sure to switch off any electrical devices in the tank, including the filter and the heater. These devices can be dangerous, as heaters can break if they are exposed to a lot of air whereas filters can get dry during the suction process.
Initiate the suction process
As mentioned earlier, you should gradually change the water in a tank, and the general rule dictates that you replace 20% water at one time. If you remove more water, you risk shocking the fish. You should place one end of the gravel siphon in the fish tank while the smaller end should be placed in a bucket. Begin the suction process so that the water from the tank starts to flow to the bucket. Let the process continue until about 1/5 of the water is removed.
Now add fresh water to the tank, which is close to the temperature of the water already in it. You should also add a de-chlorinator into the tank depending on the amount of water you are adding.
Replace all the decorations and restart the electric devices
Once the water is filled, replace all the decorations and restart the heater and the filter in the tank.