To keep tropical fish happy, it is important that you change about a third of the water in their tank every week. The simplest way is to make a siphon from an old hose or plastic tubing. Pinch the bottom tightly with your fingers and fill the tube with water from the tap; pinch the top closed and carry it to the tank. Insert one end in the tank and the other in a bucket a few feet or a metre or so below it. The water will flow automatically from the tank to the bucket. When you've removed enough water, refill the tank with water that has been sitting at room temperature for several hours.
To start the siphoning without filling the tube with water, suck sharply on the loose end of the tube before putting it in the bucket. But do this carefully, as it is easy to get a mouthful of fishy water by mistake.