Aquaculture as a growing industry faces with many limitations that can lead to reduced production (Yalsuyi and Vajargah, 2017a, b). Water pollutants, dehydration and climate change are some of these limitations (Sattari et al., 2020; Bueno et al., 2015). However, diseases can be marked as the most important cause of aquaculture growth limiter and its economic losses (Fred and Meyer, 1991). FAO (2014) reported that only acute hepatopancreatic necrosis disease in Penaeid shrimps (AHPND) results with serious economic losses in shrimp farms of China, Vietnam, Thailand and Malaysia in 2010. Also, Kubitza et al. (2013) reported that about 15% of the mortality of farm-fish is directly related to diseases. Consequences of the disease are not limited only to mortality, they can lead to economic loss trough different ways, such as increasing feed conversion rate, reducing productivity and growth (Monir et al., 2015). According to previous studies, the globally estimated average economic loss of aquaculture due to diseases was about 5 billion of US dollars per year (Shinn et al., 2015).
Managers of aquaculture farms are constantly trying to control diseases by different methods: use of food additives ( vitamins, enzymes and antibiotics) in diet to strengthen the immune system (Overton et al., 2010; Vajargah et al., 2018; Hoseinifar et al., 2020; Hamed et al., 2021; Morshedi et al., 2021; Rashidian et al., 2021); the use of chemical disinfectants. Disinfectants are a relatively common method of prevention and management of pathogens in aquaculture (Katharios et al., 2007; Harikrishnan et al., 2021; Forouhar Vajargah, 2021). Disinfectant compounds can very vary and are used in several different shapes (Bregnballe, 2015). For example, sodium chloride (NaCl) or salt is a common disinfectant compound that it is used as a disinfectant compound in hatcheries and freshwater fish farms (Boyd and Mcnevin, 2015). Formalin solution is also a famous disinfectant in aquaculture (Hodkovicova et al., 2019). It is used to eliminate pathogen agents (Guimaraes et al., 2012). However, the use of these chemical compounds have some limitations. Formalin is highly toxic and has a stable structure remaining in the environment for a long time (Buchmann al., 2004). The results of Buchmann et al. (2004) study showed lengthy exposure (24 h) to formalin at lower concentrations caused a decrease in mucous cell density of rainbow fish (Oncorhynchus mykiss). Leal et al. (2018) stated half-life of formalin was more than 30 h during which formalin led to several damages on fish and alteration mucosal cells. The common methods didn't showed efficient on formalin removal from water environments, and the advanced oxidation processes can be a good alternative. However, these methods are expensive and sometimes haven't economic justification (Leal et al., 2018).
The development of aquaculture and the need of farms for more effective chemical disinfectants, along with restrictions on the use of chemical compounds (e.g. formalin and green malachite) led to the introduction of metallic nanoparticles (MNPs) as more effective disinfectants for use in aquaculture (Shah and Mraz, 2020; Sinha et al., 2021a, b). Metallic nanoparticles such as silver nanoparticles (Ag-NPs), cadmium nanoparticles (Cd-NPs) and copper nanoparticles (Cu-NPs) are more effective and they have a more stable structure than previous compounds and protect the environment against pathogens for a longer period of time (Yalsuyi and Vajargah, 2017; Mohsenpour et al., 2020). Thus, their applications in aquaculture are expanding rapidly (Yalsuyi et al., 2017). However, the previous studies showed that these compounds were high toxicity for fish and other aquatic organisms, Their sub-lethal concentration induced l tissue lesions such as hypertrophy and hyperplasia of gills, necrosis of liver, alteration epithelial and mucosal cells and reduce reproduction and growth of fish (Vajargah et al., 2018; Forouhar Vajargah et al., 2020). Ag-NPs can remain in natural water for several months and their half-life is more than 28 days (Zhang et al., 2019). Ag-NPs were "highly toxic" and "toxic" for rainbow trout (Oncorhynchus mykiss) eleuthero-embryo-larva and juvenile (Johari et al., 2013) showed Silver nanoparticles increased cortisol and cholinesterase in fish blood, reduce growth rate and survival rate, damaged tissue and increase stress of fish. All this suggest that direct applications in aquatic environment or aquaculture should be banned. Therefore, there are concerns about emissions of these substances and their wide distribution in nature and in a more comprehensive approach, the use of efficient and environmentally friendly compounds is an inevitable necessity (Dreher, 2004; Nel et al., 2006).
Povidone-iodine (Betadine) is a water-soluble compound. It is used as a common disinfectant around the world. Betadine is usually produced as a 10% solution of povidone-iodine, and it is used millions of kilograms each year in the world (Huschek et al., 2004; Vajargah et al., 2017). The connection of iodine to S-H and N-H groups in amino acids (i.e. cysteine, lysine) can lead to denaturation of structural and enzymatic proteins in cells. Also, iodine can react to fatty acids and nucleotides (Zawada et al., 2014). Finally, these changes in the cytoplasm and membranes of cells can lead to their mortality. Iodine has a short half-life in aquatic environments (Vajargah et al., 2017). The results of Lin et al. (2018) study showed that the half-life of iodine was about 5 h. iodine also is a strong biocide that even has good performance at 0 ○C. It doesn't damage metal and/or plastic tanks. Iodine has better performance as compared with chlorine compounds. Vajargah et al. (2017) reported that povidone-iodine (Betadine) was practically non-toxic for Common carp (Cyprinus carpio) and didn't irritate skin and mucous membranes at sub-lethal concentrations (below 80 mgl− 1of povidone-iodine). However, there are few reports to mention the use of povidone-iodine in aquaculture (Chen et al., 2018). The biocide efficacy of iodine is dependent on environmental factors (Amend, 1974). The organic matter colloids can significantly reduce the antibacterial effect of povidone iodine (Rodriguez Ferri et al. 2010). The present study aims to evaluate the toxicity effect of povidone-iodine as a disinfectant solution, trough the survival chances and histology of gill tissue of Oranda goldfish (Carassius auratus), famous commercial freshwater species.