Carol,
Not being a dietary expert (please don't even ask about the "quality" of my own diet and suffice it to say I add vitamins to help compensate
) I can only try to piece together what I have read about the "fats" in fresh vs saltwater foods. The mysis that seahorses are most often fed are freshwater/brackish water creatures and are enriched to add a balance for saltwater stomachs. For octopuses, the preferred choice has been saltwater critters but many recommendations are for brackish water animals even as primary diet (fiddlers vs mithrax) because of availability and cost. But all these are recommendations about
primary diets and not occassional feedings (like your praun or crawfish) that are both accepted and relished as a special item.
All that being considered, I have an Iguana that lived on a consistent diet of less than proper food for over 6 years. She is still quite alive after 10+ years with us but good information about diet was simply not available at the time she was given to us and I followed the diet the only books and online info provided at the time (and have corrected the situation with observed improvement in overall health).
Of interest, sometime ago Roy posted a story about feeding one of his octopuses a large praun that was greedily consumed but resulted in fouling the tank the next day to the point of almost losing the octopus. His point was water quality, size of tank and how an extra large meal could be disastereous but I also question the richness of the FW food in the mix of events. When I asked Roy about that posibility he did not feel my thought was a factor.
My rambling point being that I am not sold 100% on diet theory (particularly for an animal that lives only a year) but try to totally avoid anything that is known to be bad for a critter (in the case of my Iguana, certain kinds of plant leaves are poisonous). I suspect that prauns, other freshwater shrimp and crawfish are not bad for the animal, are not good as a primary diet but are definite alternates to starvation (i.ei animal won't eat what's good for it or a shortage of desired food), sort of white bread over whole wheat kind of thing.