Anaesthetics are widely used to reduce stress, excessive movement and perception to external stimuli associated with aquaculture procedures (Inoue, Afonso, Iwama, & Moraes, 2005). As a result, since 1930 (Hoskonen & Pirhonen, 2004), anaesthetics have been used in fish to facilitate management and reduce stress (Ross & Ross, 2008), depressing the central nervous system (CNS) and reducing sensory perception, presumably increasing the welfare of fish when manipulated (Carter, Woodley, & Brown, 2011;Zahl, Samuelsen, & Kiessling, 2012). In fish, the anaesthetic is given by immersion in anaesthetic solution (Mello et al., 2012). It is absorbed by the gills and diffused into the blood, leading to the CNS (Ross & Ross, 2008). The 2-phenoxyethanol (2-Phe) (Ethylene glycol monophenyl ether) is a colourless, oily and aromatic liquid. It is believed that this agent inhibits the activity of NMDA (N-methyl-D-aspartate) glutamate receptors, reducing pain (Musshoff, Madeja, Binding, Witting, & Speckmann, 1999). Activation of the NMDA receptors increases the excitability of the neurons, resulting in the opening of a nonselective ion channel for the cations, allowing the flow of sodium and calcium into the interior of the cell and the flow of potassium to the exterior (Priborsky & Velisek, 2018). Inhibition of the NMDA receptor has been associated with an analgesic effect (Grasshoff, Drexler, Rudolph, & Antkowiak, 2006). The 2-Phe is considered suitable for use in aquaculture because of its ease of preparation, low cost, quick action and uncomplicated recovery (Pucea, Garin, & Freminet, 1989); it also presents bactericidal and fungicidal properties (Jolly, Mawdesley, & Bucke, 1972).As alternatives, essential oils (EOs) can replace synthetic products. Belonging to the Myrtaceae family, Melaleuca alternifolia has shown itself to be a good anaesthetic for use in fish (Correia et al., 2018;Hajek, 2011). Known as tea tree and widely used in traditional medicine for anaesthetic and analgesic properties, this tree, which is native to Australia, also has antifungal, antibiotic and anti-inflammatory characteristics, having terpinen-4-ol and γ-terpinene as main components (Carson,