They are eating brine, flake, mysis, pellets yet refusing nori.
Mysis is the only worthwhile food on the list...the rest are snack food. Try to portion the food accordingly, so mysis makes up the bulk.
Dry food like pellets can and should be amended with fish oil. Same fish oil as what you may already be taking....just open up a pill and soak the pellets in as much oil as they will take up before you feed them to the tank.
In a tank with a large fish population, I do like using flakes in an auto-feeder set on very low so there's a little food being added around the clock. This should be a very minor part of their diet.
Try to add some additional sea foods like fresh or whole-frozen clams. Or something with probiotics like LRS frozen food. (Though in general, processed foods like that are someone less-good than the whole-frozen foods mentioned earlier.)
I have same issue with a tomini.
That is a family of detritivores anyway, so don't sweat it! :)
They have to be trained. Make it available to them, start with small amount so whatever isn't eaten isn't polluting the water. Can take several days to weeks.
+1
Place fresh noori in the tank at least once a day. Remove the uneaten noori at the end of every day.
Not sure exactly what did it but one cold morning
Temperature swings can actually do it, but was that just coincidental wording? :)
What was your feeding system like? Are you more or less hooked up into a @Paul B -style system where there are some live and/or whole-frozen foods making up the bulk of the diet?