The diff your talking about is amino acid profile.
Fresh verses marine creatures have a different profile, it is better to feed marine fish marine sourced foods, but that is not always possible.
Protein varies very little, and that is not an issue, free fatty acids is of great concern, simply because the sourced foods usually have to higher levels, this must be accepted and taken into account when feeding, supplementary feeds of gut loaded brine shrimp a few times a week is very beneficial.
Enrichments should consist of different foods for the adults, Spiralina, (lots of vit C a very good antioxidant, also most all known trace elements, as well as many other vitamins and minerals) Chlorella (much like) and if you can get any, astaxanthin, ( a very oil based antioxidant) if this is not available, naturose, or even failing than, some regular feeds of krill. spiralina
Also important is freshness, never keep mysids for more than about two months, 3 is maximum IF you know its history, ie, freezing times from death and to terminal temp, holding temps-any fluctuations, IMO play it safe, 2 months for frozen product maximum holding time. Also never thaw and hold over or refreeze.