I believe that whole Capelin has a good nutritional profile - but I can't speak about the roe itself. The Monterey Bay Aquarium uses it in numerous diets, especially with the penguins. My understanding is that it has a lower fat content then what many penguins are dining on in the wild. This works out well for animals in captivity that aren't using up energy hunting food. Capelin is sustainably fished, so I think it is a good option.
I have fed the roe (taken from whole fish) out to numerous tanks and it definitely prompts a great response.
+1 to capelin being lower in fat compared to, say, herring. I don't know how it compares to tropical fish though. I've also never been able to find any info on the fat makeup of the eggs, but several nutritional facts pages like this one indicate that they are pretty low in fat compared to, say, blackworms.
http://www.clearwater.ca/en/home/products/masago/masago1/nutrition.aspx
Our penguins won't eat female capelin. They can actually distinguish them on sight right away and ignore them for some reason. Size? Taste? Dunno.
Anyhow, capelin roe is sold as "masago" at some asian food stores as a sushi garnish, but it's dyed and seasoned. Finding it undyed and unseasoned is tough, but I managed to track down packages of frozen female capelin at an Asian food store called Ranch 99 market here in the Bay Area:
Females can be distinguished by the silver (instead of beige) bellies, smaller size, and reduced anal fin. These babies each have about a cherry sized chunk of eggs inside of them. Apparently you can buy processed masago and rinse out the dye and seasoning like in the linked thread above but I've never tried it.
My anthias and tons of other fish love them. I feed them to cardinals, chromis, butterflyfishes, etc. One of the big upsides is that the food is nice and encapsulated and won't foul or cloud your water. Once they're thawed you can keep them in saltwater in the fridge for several days without going bad.