Add nitrifying bacteria culture solution every time the fish tank changes water. In fish tank culture, water should be raised first, and the key to water raising is to establish nitrification system. Nitrification system is composed of nitrifying bacteria, which are generally divided into two types, one is nitrite bacteria, which can convert ammonia nitrogen into nitrite, and the other is nitrite bacteria, which is responsible for converting nitrite into nitrate. Because changing water will cause some damage to nitrification system, it needs to be re-cultured.

1. Frequency: The first thing to pay attention to when changing water is frequency. Usually, there is no fixed period for changing water, which should be decided according to the water quality. If the small fish tank is poorly filtered, it often needs to be changed once every three to five days. If it is a large fish tank and the filtering effect is very good, it is no problem to change it once every two weeks.

2. Water quantity: Besides frequency, water quantity is also a point to pay attention to. If you change too much water every time, the new water will stimulate the fish and even produce stress reaction, but if you change too little water, it will often fail to improve the water quality. Therefore, the amount of water changed each time should be about one quarter or one fifth, and no more than one third at most.

3. Temperature: In order to ensure the health of fish, the temperature before and after changing water should be consistent. If the temperature changes too much, fish cannot adapt. Therefore, before changing water, measure the water temperature in the fish tank first, and then allocate new water according to this temperature to ensure that the temperature difference between the two is within 1 ℃.