Article in English: When flapper trouble isn't the cause of a running toilet, the next most common cause is the water level. When the water level is too high, water will constantly drain into the overflow tube. Be sure to fix this problem as soon as possible to prevent water from spilling out onto the floor causing major water damage if the toilet gets clogged.  With the water running and the tank full, look at the overflow tube. This is an open tube in the middle of the tank that connects the tank and the toilet bowl. Check to see if water keeps draining into the tube. If that’s happening, you can adjust the water level by lowering the float. Water comes into a toilet tank via a fill valve. The fill valve has a float on it that rises or lowers with the water level. The float height is what tells the fill valve to shut off when the tank is full. Therefore, you can lower the water level in the tank by adjusting the height of the float. There are 2 main types of floats:  A float ball fill valve will have a long arm attached to the fill valve, and at the end of the arm there will be a rubber ball-shaped float. Shake the float ball to determine if there is water inside it. If so, replace it. A float cup fill valve will have a small circular cylinder wrapped around the body of the fill valve. The cylinder, or float cup, slides up and down on the fill valve shaft, and its height determines the water level. On top of the fill valve, there will be a screw attaching the float arm to the fill valve. When you turn this screw, you can adjust the height of the float. With a screwdriver, turn the screw a quarter turn counterclockwise to lower the float.  Flush the toilet and let the water in the tank refill. Check the water level. Ideally, the water level should be 1 to 1.5 inches (2.5 to 3.8 cm) below the top of the overflow tube. Continue adjusting the screw by quarter turns until the water level is right. The float on a float cup fill valve is adjusted in much the same way. There will be an adjustment screw on the top of the fill valve....

Summary in Vietnamese:
Kiểm tra mực nước. Xác định loại bóng phao mà bạn đang sử dụng. Hạ thấp bóng phao đối với loại phao tròn. Đối với bóng phao dạng phễu cũng vậy. Kiểm tra ống cấp nước nếu toilet chảy nước liên tục.