Generally speaking, the water is green, and it is all moss growing on the inner wall of the fish tank, and the water does not change color. I suggest manual cleaning. This tool is sold in the fish market. Cleaning with chemicals is not recommended. Chemicals are still harmful to fish. After cleaning, change the water from 1/3 to 1/2. Pay attention to the fact that fish tanks can't be exposed to direct sunlight, which is easy to grow moss and is not conducive to the growth of nitrifying bacteria and most fish. Fish also like to avoid light. If there are aquatic plants, you can also buy a lamp tube, which will not grow green moss, but also allow plants to photosynthesis, which is also beautiful and conducive to viewing.
If the water itself turns green, it means there are algae in the water. I suggest you raise a new tank of water, which is the same as the red tide in nature. You can't get water. However, this situation rarely happens in domestic fish tanks.