The aim of this study was to develop a self-emulsifying system (SES) with practical applications in fish farming. In particular, the lipid vehicle was developed in order to deliver oxytetracycline hydrocloride to rainbow trout (RT) and European sea bass (ESB) so as to improve the drug’ s oral bioavailability. The developed formulation was assessed in comparison to an aqueous solution working with two fish species (one fresh-water and one salt-water), after oral administration with a gastric probe. Results indicated an enhancement of bioavailability of 5.86 and 5.41 times over the aqueous solution, in RT and ESB, respectively. SES was then used to prepare medicated feed containing the formulation. The pharmacokinetic of this feed was evaluated after oral administration and compared to that of commercial OTC medicated feed. The bioavailability of OTC delivered in SES was 3.2 times higher in RT and 2.7 times higher in ESB, than OTC supplied by commercial medicated feed. This bioavailability enhancement was confirmed when RT were fed by classical administration in fish tanks of single and repeated administration of OTC delivered in SES and commercial medicated feed, attesting to better absorption of the SES formulation.