RT-PCR can help if you know specific target which are impacted by your chemical toxicant (to compare the modification before and after the adding of the plant product). But RT-PCR only shows the modification which impact the transcription of genes or the regulation of the RNAm, but the product can have none impact on the RNAm level but have one on the protein level (or a modification of the RNAm level can show none impact on the protein level and therefore, no effect on the fish). Consequently, a western blot or FACS analysis is necessery to confirm a variation. I hope i could help.

Thanks Maureen and Romain for helping me out.