Recirculating aquaculture systems (RAS) are used in home aquariums and for raising fish when water exchange is limited. These systems use biofiltration to reduce harmful ammonia levels. Other types of filtration and environmental controls are often needed to keep water clean and create a good home for fish. A major advantage of RAS is the ability to use less fresh, clean water while still supporting fish health. For commercial use, RAS must operate efficiently by keeping many fish in a small space. Scientists are currently studying whether RAS can be a practical method for raising large amounts of fish.

RAS water treatment processes

Water treatment processes are used to keep water clean in fish farming systems. These steps often happen one after another or at the same time. After leaving the fish tank, water is first treated to remove solid waste. Then, it moves to a biofilter, where ammonia is converted into nitrate. Next, the water is degassed and oxygen is added. After that, the water may be heated or cooled, and finally, it is sterilized. Each of these steps can be done using different methods and equipment, but all are needed to create a healthy environment for fish to grow and stay healthy.

All recirculating aquaculture systems (RAS) depend on biofiltration to change ammonia, a waste product from fish, into nitrate. Ammonia is harmful to fish when its concentration is above 0.02 mg/L. Special bacteria, such as Nitrobacter, Nitrococcus, Nitrospira, and Nitrospina, help convert ammonia into nitrite and then into nitrate. Although nitrite is usually quickly turned into nitrate, if this process stops, nitrite levels can rise and harm fish. High nitrite levels also signal that the biofilter may be failing. Nitrate is the final product of this process and is the least harmful nitrogen compound, with safe levels in freshwater above 1,000 mg/L for 96 hours.

A biofilter provides a surface for bacteria to grow, forming thick layers of biofilm inside the filter. Water is pumped through the filter, and bacteria use ammonia as energy. In recirculating systems, daily water changes are often used to manage nitrogen levels. To keep the biofilter working well, stable conditions and regular maintenance are needed.

In addition to treating liquid waste, solid waste must also be removed. This is done by collecting and flushing solids out of the system. Removing solids helps reduce bacteria growth, oxygen use, and disease spread. The simplest method is using a settling basin, where slow-moving water lets particles sink to the bottom and be removed manually. However, this method is not practical for RAS systems that need a small space. Common RAS methods include sand or particle filters, where solids get trapped and are cleaned out regularly. Another method uses a mechanical drum filter, where water flows over a rotating drum that is cleaned with pressurized water. For very small particles, a protein fractionator may be used with or without ozone.

Adding oxygen to the water is essential for high fish production. Fish and biofilter bacteria need oxygen to grow and process food. Oxygen levels can be increased through aeration or oxygenation. Aeration uses air pumped through devices like air stones to create bubbles, which allow oxygen to dissolve. However, this method is less efficient because it requires high pressure and takes longer. Oxygenation uses pure oxygen pumped into the water, which dissolves more quickly. Careful planning is needed to match oxygen supply with the system’s demand.

In all RAS systems, pH must be carefully monitored and controlled. The first step of nitrification in the biofilter uses alkalinity and lowers the system’s pH. Keeping pH within a safe range (5.0–9.0 for freshwater) is important for fish and biofilter health. pH is often adjusted by adding lime (CaCO₃) or sodium hydroxide (NaOH). Low pH increases dissolved carbon dioxide (CO₂), which can be toxic to fish. CO₂ can also be removed through degassing in a packed column or with an aerator, especially in systems using oxygenation.

Each fish species has an ideal temperature range. Warm-water species, like Tilapia and barramundi, prefer water at 24°C or higher, while cold-water species, such as trout and salmon, prefer water below 16°C. Water temperature affects dissolved oxygen levels, with higher temperatures reducing oxygen availability. Temperature is controlled using heaters, heat pumps, chillers, or heat exchangers. These tools help maintain the best temperature for fish growth.

Disease outbreaks are more likely in RAS systems with high fish densities. Outbreaks can be reduced by using multiple independent systems within the same building and preventing water contact between systems through cleaning equipment and personnel. Ultraviolet (UV) or ozone water treatment systems also help by reducing viruses and bacteria in the water. These methods lower disease risk for stressed fish and reduce the chance of outbreaks.

Advantages

• Uses less water than raceway or pond aquaculture systems.
• Requires less land because more fish can be raised in the same area.
• Can be placed in different locations and does not need a big, clean water source.
• Produces less wastewater.
• Provides better protection against diseases and makes it easier to handle disease outbreaks.
• Allows close monitoring and control of the environment to improve production efficiency. It is also not affected by weather or changing environmental conditions.

Disadvantages

High initial costs for materials and buildings.
• Expensive to run because of high electricity use and regular maintenance.
• Requires workers with special training to manage and use the system.
• Produces more greenhouse gases than non-recirculating aquaculture systems.

Special types of RAS

Combining plants and fish in a recirculating aquaculture system (RAS) is called aquaponics. In this system, ammonia from the fish is changed into nitrate, and the plants also take ammonia out of the water. In aquaponics, the fish provide nutrients to the plants, creating a system where waste is reduced, and resources are used efficiently. Aquaponics allows for growing and selling more than one type of crop. Some people have different opinions about whether RAS waste can safely support plant growth in aquaponics. When RAS farms are changed to aquaponics, these changes should not be stopped because the nutrients in RAS waste are safe and enough to help plants grow. Using RAS waste in aquaponics is encouraged.

Home aquariums and inland commercial aquariums are types of RAS where water quality is carefully managed, and fish are kept in low numbers. In these systems, the main goal is to show fish, not to grow food. However, filters and other water treatment methods are still used to reduce the need for water changes and to keep the water clear. Like in traditional RAS systems, water must be removed regularly to stop harmful chemicals like nitrate from building up. Coastal aquariums often replace large amounts of water and are usually not considered RAS because they are near a large source of clean water.