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Biogas fertilizer is an organic substance produced by sealing farmyard manure and animal waste, as well as various types of straws and garbage in biogas digesters and subjecting them to high-temperature fermentation. It not only provides biogas (methane) for lighting and fuel, but also serves as a feed source for fish farming. The technology for using biogas fertilizer to raise fish is introduced as follows. I. Pond conditions: There is an adequate supply of water, the water quality is good, drainage and irrigation are convenient, the surrounding environment is favorable with good ventilation and sunlight, and transportation is easy. The pond area ranges from 0.333 to 0.666 hectares, with a water depth of 1.5 to 2 meters. Fish-proof devices are installed at the inlets and outlets to prevent fish from escaping. II. Stocking of fry (1) Selection of fry. Choose fish that are plump, free from diseases and injuries, have a bright and vivid color on their bodies, complete scales, and swim actively. The fish species used for stocking should be of larger size, as larger fish have stronger disease resistance, a higher survival rate, and faster growth. The grazing season should start early, aiming for early release, early feeding, and early growth. (II) Stocking species and density. The species used for stocking should mainly consist of filter-feeding fish such as silver carp and bighead carp, along with an appropriate amount of omnivorous fish. Generally, silver carp account for 65%, bighead carp for 15%, while omnivorous fish such as carp, crucian carp, and tilapia each make up around 7%. 700 to 1,000 fish fry with a length of 16 cm are stocked per 0.067 hectare. (III) Pond disinfection. Before introducing fish fry into the pond, it must be disinfected; 70–100 kilograms of quicklime per 0.067 hectare should be used, and it should be spread evenly throughout the pond on a sunny day in order to eliminate pests and pathogenic bacteria. (IV) Stocking of fish fry. During fishing and transportation, fish can suffer scratches to varying degrees, which makes them susceptible to infections; moreover, untreated fish can introduce new pathogens into ponds that have already been disinfected. Therefore, before releasing the fish fry, they should be soaked in a 2.5% salt solution for 10 to 15 minutes. III. Requirements for raising fish using biogas slurry (1) The fish pond should be integrated with the biogas digester. Generally, a 100-square-meter fish pond is paired with a biogas digester of 0.2–0.3 cubic meters, and several such digesters should be installed so that they can be used in rotation. (II) Fermentation time in the biogas digester. Generally speaking, after fertilizers are added to the biogas digester, after about 1 month, the levels of readily available nitrogen, phosphorus, and other nutrients in the biogas fertilizer reach their peak; applying this fertilizer to fish ponds at this time yields the best results. (III) Application rate of biogas slurry fertilizer. Swamp fertilizer is a fast-acting fertilizer; when applying it, it is necessary to follow the principle of using small amounts but frequently. Generally, it should be applied once a day, at a rate of 5 kilograms per 100 fish. If the weather is hot and the water clarity is below 25 centimeters, fertilization can be carried out every other day or every few days. During use, the transparency of the water color should be no less than 20 centimeters; in mid-summer and early autumn it should be kept below 25 centimeters, while at the end of autumn 25 centimeters is an appropriate value. Manure application should be carried out on sunny days between 8 and 9 a.m.; it should not be applied on cloudy or rainy days, nor when fish are floating to the surface. Apply less fertilizer when fish are sick. IV. Advantages of using biogas slurry for fish farming (1) Stable and suitable pH level. Pond water to which biogas slurry is applied can remain neutral at all times. The pH value of biogas fertilizer is 7.2–7.8, which is the same as the pH requirement for high-yield fish ponds. The continuous addition of biogas fertilizer helps maintain the pH level of the fish pond water at a neutral level, which is suitable for the growth and development of fish. (II) The incidence of disease in pond fish is low. After biogas fertilizer is sealed and fermented at high temperatures, the pathogenic bacteria and parasites in the manure are killed. Moreover, due to the low oxygen consumption of biogas slurry, the incidence of disease among fish in the ponds can also be **reduced, thereby significantly increasing their survival rate. Biogas fertilizer is rich in nutrients, containing nitrogen, phosphorus, potassium, and inorganic salts. After the application of biogas fertilizer, the water color in the pond turns green within 3–5 days; plankton reproduce rapidly, resulting in more sunlight and heat being absorbed. This leads to an increase in oxygen levels and water temperature, which is beneficial for the growth of fish. (III) Comprehensive utilization of livestock waste. Using biogas slurry to raise fish provides a wide supply source, low costs, and high efficiency. At the same time, by making full use of fertilizers, it can replace large amounts of concentrated feed, reduce environmental pollution, and bring significant economic, social, and ecological benefits.