Sustaining good water quality in aquaculture ponds is vital. Without an aerator, the dissolved oxygen in ponds comes primarily from mass transfer at the water-ambient atmosphere interface. As sediment can seriously affect water quality, this study used indoor experiments to examine the nutrient (nitrogen and phosphorus) release mechanisms and fluxes from sediment in aquaculture ponds with moving water but no aeration. The results showed that the ammonia nitrogen (NH₃-N) concentration in the overlying water was inversely proportional to flow velocity and that a higher flow velocity tended to result in a lower concentration in the overlying water, a steeper vertical gradient of concentration within the bed sediments, and a faster release rate from the sediments. The sediment disturbed by flowing water released more nitrate nitrogen (NO₃-N) and nitrite nitrogen (NO₂-N) into the overlying water and NO₂-N could become oxidized into NO₃-N. In still water, NO₃-N was released gradually and some anaerobic NO₃-N was nitrified into NO₂-N. Phosphorus release from the sediments was controlled by the adsorption–desorption balance, with the phosphorus concentration in the overlying water dropping gradually to a steady value from its initial maximum. The relationship between NH₃-N release flux and flow rate is described by a cubic function.

Traditional aquaculture can cause serious environmental pollution. Biofilm and biofloc technology have the potential to limit aquaculture pollution. An outdoor experiment was conducted to evaluate the combined effects of eco-substrates and carbon addition on water quality, fish performance and nutrient budgets in the pond polyculture system. In the treatment group, the total ammonia nitrogen, nitrate nitrogen, nitrite nitrogen, and total nitrogen of the water were significantly lower compared to the control group (p < 0.05). The growth performance of H. molitrix and A. nobilis, including the final individual weight, specific growth rate, weight gain rate, total production and net production, was significantly higher in the treatment groups compared to the control (p < 0.05), while there was no significant difference in those of C. carpio between the two groups. Feed was the main N (>92%) and P (>95%) input during the experiment. Comprehensive accumulation was the main N (>58%) and P (>69%) output. The N and P feed input and comprehensive accumulation output declined in the treatment group. The N utilization efficiency in the experimental group increased slightly (p > 0.05), while the P utilization efficiency in the experimental group was significantly higher compared to the control (p < 0.05). Therefore, the application of eco-substrates and carbon addition can increase water quality, improve fish growth, and promote nutrient utilization efficiency in pond polyculture systems.

Oxygenation efficiency has gained more and more attention in water bulk with a fine bubble aeration diffuser system operating. To enhance the oxygenation efficiency, one of the important researches is to explore the respective impact of different sources and the key factors on dissolved oxygen mass transfer, which will in turn be better for the design and operation of the fine bubble aeration diffuser system. A series of laboratory experiments for re-oxygenation were carried out in a glass tank, the fine bubble aeration diffusers were used to generate abundant fine air bubbles. There were two regions, i.e. the air bubbles and the non-bubbles zones in the experimented water bulk. According to the mass balance theory, the bubble-water surface mass transfer, turbulence diffusion mass transfer, global mass transfer and their oxygen utilization rates were achieved. The results showed that the values of oxygen volume mass transfer coefficient in air bubbles zone are smaller than the values of global oxygen volume mass transfer coefficient. The global oxygen utilization rate is greater than the oxygen utilization rate of bubble-water surface mass transfer. The impact of key factors on the global mass transfer are the bubble rise velocity, water velocity, submerged water depth, air flow rate, bubble diameter and water volume. The important factors improving aeration efficiency are the bubble rise velocity, bubble diameter and water velocity.