Oh i know a lot about why copepods are such great foods since our last reef club meeting i had that what we talked about for 2 and a half hours....
Marine animals contain very high amounts of omega-3, they have evolved into animals that need a high omega-3 diet. Phytoplankton consists of the highest amount of omega-3 and in the ocean, they are at the bottom of the food chain. Filter feeders eat this phytoplankton, this includes zooplankton (copepods are a type of zooplankton and therefor have a high omega-3 content) worms, clams etc etc. Then higher organisms eat this zooplankton like fish, corals etc and the zooplankton then transfers its omega-3 into the fish. Then we eat the fish and it transfers to us etc etc.
Phytoplankton is a green/brown suspended algae, green water is a tank with phytoplankton. Look in coral reef parts of the ocean and i would doubt you would see a dark green, thats because there is so many things eating this phytoplankton and so much sun feeding the phytoplankton. Since phyto has omega-3 and everything somehow eats phyto, obviously marine life would be high in omega-3. Now look at a stagnant or slow moving body of freshwater, notice how lots of them are full of green algae even with life like insect larvae, fish etc? This is because omega-3 is not the basis of freshwater fish, it is not their most important diet and there arent many freshwater filter feeders that feed on phytoplankton than there are saltwater. So most freshwater organisms will have a low content of omega-3.
Most frozen foods we buy comes from freshwater, and if you look at your box of mysis, it came from freshwater, i dont think there are any major distributors of saltwater mysis. This freshwater mysis is low on omega-3, it is plenty for freshwater organisms since tehy dont need much omega-3, but for saltwater organisms, it isnt enough arguably. Therefor it is always best to feed saltwater ocean zoo-plankton that is rich in omega-3 since most of them are herbivores that only feed on phyto. Seeing that mandarin fish eat pods in the wild and most of those pods eat phyto and phyto is rich in omgega-3, then obviously the mandarin should need lots of omega-3. So if you cant afford a pure diet of pods like tigger pods or arcti pods etc, then the second best thing to do is to enrich your frozen foods for it.
Im not sure why brine shrimp is so low on it though, they are filter feeders that can feed off phytoplankton and they are saltwater organisms, but i am unsure why they are so low in nutrients.
Feel free to ask questions, and i made a post about this that goes into detail more a while ago.
If i were you i would skip the brine shrimp and feed mysis, although fish love omega-3 and need it, obviously they need other nutrients with it as well, mysis is generally more nutritious than brine shrimp, now if you enrich the mysis with garlic and selcon (a commercially available omega-3 enricher) than it will be very very nutritious for your mandarin and also for all your other fish.
But in the end if it refuses mysis, then use brine, its probably not as good as mysis but its better than a starving fish