The reason for changing the water is to keep water values within the normal range, i.e. a pH value above 5.0 and a nitrate value less than 50 mg.
Clean your hands with a brush under running water (without using soap). This will remove contaminants such as the remains of cream which may be on your hands and which should not get into the water of the aquarium under any circumstances.
Prepare cleaning utensils which you should only use for cleaning the aquarium and changing the water. It is important that no remains of cleaning agents are in the bucket or on the other cleaning utensils since these contaminants can seriously injure your fish! Useful utensils for cleaning and changing water are: bucket, scraper toothbrush, rubber gloves and a thin hose for suctioning off water and dirt.
Placing pumps and heaters in the correct position. Before you start changing the water, slide the pumps towards the bottom to reduce the amount of water flowing through. The pumps should not run dry since they would be damaged irreparably. You should also ensure that the heater is always under water or turned off since this would burn out if exposed to the air. When you switch off the heater for cleaning, don’t forget to switch it on again when you have finished cleaning!
Create a vacuum in the hose so that the water can flow out of the aquarium into the bucket through the hose. Remove 25% of the stale water by suctioning out the dirt and debris in the substrate. Always make sure that the end of the hose is in the bucket and that the bucket does not overflow! Clean out algae and other residue inside the aquarium with a brush and scraper, avoid fast movements to not spook the fish.
Adding fresh water,
Pour the fresh dechlorinated water carefully from the bucket (which should be used solely for this purpose) into the aquarium. The new water should be the same temperature as the remaining aquarium water. Add a plate to the bottom of the aquarium to avoid disturbing the substrate. Repeat the process until the former level of the water is reached. Don’t forget to turn back on the pumps and the heater after you’ve finished your water change.