**4. Conclusions**

This study demonstrated an effective approach to utilizing the demonstrated fermented substrate for aquafeed, starting with fish and lemon peel wastes, allowing the conversion of both animal and vegetable food wastes into an added-value product.

The final fermented product is low in spoilage microorganisms and rich in healthy microorganisms, representing a healthy final substrate enriched by added value.

The microorganisms' ability to feed on fermenting fish waste that is supplemented by lemon peel was confirmed by the protein level's increasing during the process, up to 48.55%, making these wastes an excellent raw material for aquafeed production via *Lactobacillus reuteri* and *Saccharomyces cerevisiae.* In fact, the final protein and lipid contents represent adequate levels in the resulting aquafeed, reducing the problem of a lack of protein sources for aquaculture by encouraging the conversion of fish waste and lemon peel into feed.

This study pointed out the possibility of setting up a fermentation process based on the simultaneous addition of two different microorganisms, reaching a plateau after 96 h. Further studies are in progress for converting the resulting fermentation product into pellets and for testing the effect of the final product on the growth and immune response of fish from aquaculture and, consequently, in human consumers.

Finally, additional work will be needed to further optimize production to facilitate future larger-scale production, also evaluating it from an economic point of view.

**Author Contributions:** Conceptualization, A.T., V.L.T., A.G.P. and G.D.B.; methodology, A.T., V.L.T., A.G.P. and G.D.B.; validation, A.T., V.L.T., A.G.P. and G.D.B.; formal analysis, A.T., V.L.T., A.G.P. and G.D.B.; investigation, A.T., V.L.T., A.G.P., E.R., R.V. and R.R.; resources, A.T., V.L.T., A.G.P. and G.D.B.; data curation, A.T., V.L.T., A.G.P., E.R., R.V., R.R. and G.D.B.; writing—original draft preparation, A.T., V.L.T., A.G.P., E.R., R.R. and G.D.B.; writing—review and editing, A.T., V.L.T., A.G.P. and G.D.B.; supervision, A.T. and G.D.B.; project administration, A.T. and G.D.B. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest.
