Microcystis sp. is a harmful cyanobacterial species commonly seen in earthen ponds. The overgrowth of these algae can lead to fluctuations in water parameters, including DO and pH. Also, the microcystins produced by these algae are toxic to aquatic animals. This study applied hydrogen peroxide (7 mg/L) to treat Microcystis sp. in a laboratory setting and in three earthen pond trials. In the lab we observed a 64.7% decline in Microcystis sp. And in our earthen pond field experiments we measured, on average, 43% reductions in Microcystis sp. cell counts within one hour. The treatment was found to eliminate specifically Microcystis sp. and did not reduce the cell count of the other algae species in the pond. A shift of the algae community towards the beneficial algae was also found post-treatment. Lastly, during the pond trials, the gill status of Tilapia and Giant tiger prawn were not affected by the H₂O₂ treatment suggesting this may be a good mitigation strategy for reducing cyanobacteria in pond aquaculture.

Algal blooms can lead to low dissolved oxygen and fluctuating pH, and the toxins produced by some species can be toxic to aquatic animals. In this study, we assessed the potential of ozone nanobubble technology for reducing a diatom species, Nitzschia sp., commonly found in freshwater ponds in Hong Kong. This study suggests that ozone at a low dose of 0.025±0.003ppm was sufficient to reduce algae by 66.4 within 5 minutes of treatment. An even higher killing effect (68.2) was observed when ozone was delivered using nanobubbles for 9 minutes. A 24-hour delayed effect was also detected, with a further reduction of approximately 10 of the algae for both ozone treatments (macro and nanobubble delivery methods). In addition to controlling algae, applying ozone at a level that is not detrimental to fish may also benefit the dissolved oxygen levels in pond systems.

[This corrects the article DOI: 10.1016/j.aquaculture.2023.739932.].

Copper-based fungicides have a long history of usage in agriculture and aquaculture. With the rapid development of metal-based nanoparticles, copper-based nanoparticles have attracted attention as a potential material for prevention and control ofSaprolegnia parasitica. The present study investigated the effectiveness of copper/carbon core/shell nanoparticles (CCCSNs) and a commercial CCCSNs filter product (COPPERWARE^®) againstS. parasiticain a recirculating system. Results showed that the growth of agar plugs with mycelium was significantly suppressed after exposure to both CCCSNs powder and COPPERWARE^®filters. Even the lowest concentration of CCCSNs used in our study (i.e., 100 mg/mL) exhibited significant inhibitory effects onS. parasitica. The smallest quantity of the filter product COPPERWARE^®(3.75 × 3.7 × 1.2 cm, 2.58 g) used in our aquarium study also demonstrated significant inhibition compared with the control group. However, we observed leaching of copper into the water especially when larger quantities of COPPERWARE^®were used. Water turbidity issues were also observed in tanks with the filter material. Besides these issues, which should be further investigated if the product is to be used on aquatic species sensitive to copper, CCCSNs has promising potential for water disinfection.

Laser-induced graphene (LIG) has gained popularity for electrochemical water disinfection due to its efficient antimicrobial activity when activated with low voltages. However, the antimicrobial mechanism of LIG electrodes is not yet fully understood. This study demonstrated an array of mechanisms working synergistically to inactivate bacteria during electrochemical treatment using LIG electrodes, including the generation of oxidants, changes in pH-specifically high alkalinity associated with the cathode, and electro-adsorption on the electrodes. All these mechanisms may contribute to the disinfection process when bacteria are close to the surface of the electrodes where inactivation was independent of the reactive chlorine species (RCS); however, RCS was likely responsible for the predominant cause of antibacterial effects in the bulk solution (i.e., ≥100 mL in our study). Furthermore, the concentration and diffusion kinetics of RCS in solution was voltage-dependent. At 6 V, RCS achieved a high concentration in water, while at 3 V, RCS was highly localized on the LIG surface but not measurable in water. Despite this, the LIG electrodes activated by 3 V achieved a 5.5-log reduction in Escherichia coli (E.coli) after 120-min electrolysis without detectable chlorine, chlorate, or perchlorate in the water, suggesting a promising system for efficient, energy-saving, and safe electro-disinfection.

Microcystis spp. blooms in fish ponds can lead to fluctuations in water parameters, and the microcystin-LR (MC-LR) toxin they produce may pose health risks for fish, as well as consumers. Hydrogen peroxide (H ₂ O ₂ ) treatment has been demonstrated to reduce populations of Microcystis spp. in aquaculture ponds. This study conducted 2 trials to investigate whether these treatments result in the release of MC-LR under laboratory conditions. We applied 7 mg l ^-1 of H ₂ O ₂ to a Microcystis sp. culture and assessed the MC-LR level post-treatment. In the first trial, no MC-LR was detected in the H ₂ O ₂ treatment group nor the negative control group, while 0.933 ± 0.381 ppb of MC-LR was extracted from 2344230 Microcystis sp. cells in 5 ml of media stock in our positive control group (where all cells were destroyed). In our second trial, where we added almost 30 times more Microcystis sp. to the culture media (i.e. 64125000 cells in 5 ml of stock), 19.333 ± 0.742 ppb of MC-LR were detected in the positive control on average. At this concentration of algae, our negative control (no treatment) and our H ₂ O ₂ treatment group had 16.933 ± 0.303 ppb and 16.933 ± 0.109 ppb of MC-LR, respectively. The toxin levels were similar between the treated and untreated groups, but significantly less than the positive control group (p = 0.003). The results suggest that 7 mg l ^-1 H ₂ O ₂ treatment to Microcystis sp. did not increase the level of MC-LR in the solution, but MC-LR was also not removed by the treatment. The low-dose 7 mg l ^-1 H ₂ O ₂ treatment against Microcystis sp. bloom is unlikely to cause extra toxin (MC-LR) release to the medium.