30 small tanks usually change water every three to five days. In fact, the period of water change is not fixed. If the water quality is maintained well, the interval of water change can be longer, but it usually needs to be changed once every two weeks. If the water quality deteriorates rapidly, it needs to be changed once every three days. It should be noted that the amount of water change should not be too much, generally not more than one third of the total amount. If too much water change, it may affect the health of fish.

1. Trapping the water in advance: Before changing the water for the fish tank, the water should be trapped first. Because tap water is generally used for water change, although this water is relatively clean, it still has more chlorine, so it is necessary to remove chlorine by trapped water. Usually, after the water is connected, it needs to be put in the sun to dry for a few days, so that chlorine in the water will volatilize slowly.

2. Pumping water: To change water, you need to pump water first, find a pipe, lead it to the bottom of the cylinder, and then siphon water from it, usually about a quarter of it. If the fish tank is large and there is a lot of water to be changed, it can be pumped with a water pump, which is more efficient. After pumping, slowly add new water from the cylinder wall.

3. Precautions: In order to ensure the health of fish, the temperature before and after changing water must be consistent. Usually, before changing water, it is necessary to measure the water temperature first, and then allocate new water according to the measured data. The temperature difference between the two should be controlled at about 1 ℃, so that fish can adapt.