For fish farmers, there are four kinds of water: first, fresh water, that is, tap water that has just been aired or well water that has been newly drilled. Although this kind of water is clean, it is very different from the environment in which fish live in nature. Because there is no nitrifying bacteria community in the water, fish excreta and scattered food residues decompose into ammonia after corruption, and fish are easy to be poisoned. Second, the old water, light green or amber, is rich in humus and beneficial microorganisms and algae, and has established a good ecological circulation system. After nitrogen cycle, it is decomposed into nitrate which is harmless to fish. This kind of water is extremely beneficial to the growth of fish. The third is green water. There are too many organic matters in the water, cyanobacteria, green algae and brown algae multiply in large numbers, and the content of bacteria and microorganisms increases sharply. The water is thick green, and sometimes it smells bad, which can easily lead to the death of the whole tank of fish. The fourth is to return to clear water, also called biting clear water. It is because there are too many algae and microorganisms in the green water, which consumes the oxygen in the aquarium, causing the death of algae and aerobic bacteria, and turning the water into stagnant water without oxygen and a large number of anaerobic harmful bacteria.

It can be seen that not all water is suitable for goldfish's life, and the most suitable water for goldfish's growth is old water. Hydroponics is the process of changing new water into old water.

At present, most of the water we use for fish farming is tap water, so the first step of water cultivation is to remove the toxic substance-chlorine from tap water. The method of chlorine removal is relatively simple. After the water is filled, let it stand for 2-3 days, or dry it 1 day (in case of urgent need, add 0.5% per 100 kg of water). 63g of baking soda, namely sodium thiosulfate, can be used immediately).

After the fresh water is dried, it should be put into a tank for filtration, so that the water pump can turn the still water into flowing water, and then put the fish into the tank. "Broken tank" is to put a few cheap, strong and easy-to-raise fish into a new tank. After a few days, fish excrement will be decomposed into ammonia in water, which is the same as adding ammonia to tap water. Too much ammonia will kill fish, so the most important thing to raise water is to remove ammonia from the water. Usually we use nitrifying bacteria to remove ammonia from water. Nitrifying bacteria can be added to water, but they need attachments to survive in water. If there is no suitable attachment, it is not good to add more nitrifying bacteria. They will die in water, and nitrifying bacteria usually attach to filter cotton or bottom sand. In fact, running water will naturally produce nitrifying bacteria in a few days, which is also the most commonly used method to cultivate nitrifying bacteria. The cultivation of nitrifying bacteria is the second step of hydroponics. After the fish enters the tank, it will discharge ammonia and other wastes into the water, and the ammonia concentration in the water will increase, so the nitrifying bacteria in the domestic water can decompose ammonia (note: it takes a week or more for nitrifying bacteria to form, so the fish will be seriously injured during this time). When the nitrifying bacteria community is formed, the ammonia concentration will be reduced to zero.

However, it is not that simple, because nitrifying bacteria can only convert ammonia into nitrite, but it is also toxic to fish! So we should understand that the concentration of nitrite is increasing, while the concentration of ammonia is decreasing. So don't think that you can sit back and relax after the ecology is established. In order to reduce nitrite, it is necessary to change water regularly and quantitatively to reduce its concentration, so as to maintain a relatively stable ecology and keep the water in an old state. Old water is what we pursue, and green water is also good for goldfish. For example, green water can darken the color of goldfish, but there are too many algae in this water to see fish and it is not suitable for viewing. Moreover, when there are too many algae, the fish will lack oxygen, so we should pay attention to controlling the algae content in the water when raising goldfish with green water.