Abstract: Foam fractionation is a widely employed separation technology in seawater recirculating aquaculture systems (RAS) for the efficient removal of fine particulate matter. However, it was often deemed impractical for freshwater RAS due to the absence of electrolytes leading to low surface tension and large bubbles with the diminished foam formation. On the one hand, the ammonia excreted by the farmed organisms is oxidized by nitrification, which leads to nitrate accumulation and pH reduction. The addition of alkaline substances is commonly employed for maintaining the pH of water. Consequently, the increase in electrolyte concentration in freshwater contributes to the effectiveness of foam fractionation to some extent. On the other hand, ozone can oxidize particulate organic matter to yield surfactants that lower the surface tension of bubbles and enhance the process of foam fractionation. However, the solubility and the mass transfer efficiency of ozone are not high. Foam fractionation requires gas-liquid separation to generate foam, and ozone-containing gas can escape from the gas-liquid interface of water, causing low ozone utilization and increasing costs. Foam fractionation requires a lot of bubbles, while ozone also needs to be dissolved into the water by the mass transfer of bubbles. Therefore, bubbles are the key to the coupling of foam fractionation and ozonation. In order to promote the efficient application of the combined treatment in the freshwater RAS, effectively improving the overall ozone utilization rate within the closed-loop system, it is therefore necessary to generate bubbles through the combination of Venturi tube and water pump, applying the negative pressure during its operation to recirculate the ozone-containing gas overflowed from the gas-liquid interface. In this study, DN25 and DN40 Venturi tubes of polyvinylidene fluoride were combined with four pumps with powers of 370, 650, 900 and 1500 W to generate bubbles, respectively. The bubble size distribution was obtained from a high-speed camera combined with the image processing. Considering the inlet flow rate and gas holdup, the suitable combination for freshwater foam fractionation was determined. The results show that most bubbles generated by DN25 Venturi tube were 1.5～4.5 mm. With the increase of pump power, the proportion of relatively small bubbles gradually increased. However, according to the demand of foam fractionation, DN25 Venturi tube is recommended to combine with the pump of 370 W. Its Sauter mean diameter (SMD) was 3.802 mm, inlet flow rate was 24.94 L/min, and the gas holdup was 0.095. For DN40 Venturi tube, the inlet flow rates under 370 W and 650 W were significantly smaller than those under 900 W and 1500 W (P<0.05). For water pump of 1500 W, the size of most bubbles was 2.0～4.0 mm, SMD was 3.332 mm, and the gas holdup reached 0.240, which was higher than other pumps with low powers. DN40 Venturi tube is recommended to combine with the pump of 900 W or 1500 W. In order to reduce the electric power consumption of bubble generation, the pump of 1500 W was used to provide water flow to two paralleled Venturi tubes. The proportion of bubbles with the size smaller than 2.5 mm generated by DN25 and DN40 or DN40 and DN40 was more than 50% and the size smaller than 2.0 mm was more than 25%; the SMD was also relatively decreased. At the same time, the gas holdup reached 0.273 for the combination of DN25 and DN40, and 0.313 for the combination of DN40 and DN40, which were higher than that of a single Venturi tube. It is conducive to the construction of an efficient and energy-saving freshwater foam fractionation for the practical applications. In addition, the combined equipment was constructed for the treatment of actual aquaculture water, demonstrating that the average removal efficiencies of color, turbidity, and nitrite were 79.27%, 87.05%, and 46.69%, respectively. This study provides key parameters for the design of the combination treatment of freshwater foam fractionation and ozonation.