If you had any hope of genetic analysis, the formalin will have negated that. Even though formalin is the typical preservative used, over time, even it will begin to degrade your samples. Museums that I work with, especially for long-term storage, will replace samples put in formalin/formaldehyde long ago with 70% ETOH, after multiple washes with water to dilute out the formalin; even after doing that procedure, periodically the ETOH will have to be replaced with fresh ETOH, as there will still be formalin/formaldehyde leaching out of the tissues. For new samples, they go immediately into 70% ETOH for storage and future examination, with a bit of tissue from the sample into 95% ETOH for genetic analysis. For creatures as small as copepods, you would have to move pretty quickly on your dissections (you don't need to wait long if they are in formalin--and they won't get any stiffer with more time) and you will likely have to generate your own very fine dissecting needles for teasing out structures or the removal of mouth parts, which often turn out to be the best features for species identification. I made fine dissecting needles by inserting REALLY fine wire or a stiff, but really thin hair into a small wooden stick. Time-consuming, but almost the only really effective dissecting tool for a tiny copepod. I learned this from one of my graduate professors, Dr. Ju-Shey Ho, who was an expert on copepods, particularly parasitic ones, years ago (sadly he passed away some years ago). Patience, a steady hand and numerous fine needles, working on the microscope is all you can really do.

Thank you Mx./Dr. Horvath for these. I appreciate your answer. The samples are currently in absolute ethanol and I am identifying the samples as much as possible. I have a specialized needles for the dissection.