It isn't the zoo/aquarium to make your top priority when you're in Denmark (or Scandinavia as a whole), partly because of its quite isolated location and quite limited array of different animal types like Condor mentions - but if you happen to be in Hirtshals (like if you have to take a ferry to Norway), it is surely worth a visit. And if you're interested in the maritime life of Denmark, it's probably the best place to go in the world.
I've been there many, many times (my grandparents live in the town of Hjørring 15 km away, and when I was younger and visited them, we always used to go there), and yeah, I would recommend it. It's very educating in all aspects of the North Sea, fishing and the sorts, and you can see most common fish species of the North Sea, including two nice specimens of sunfishes. They've completely rebuilt half of the place since I visited last time, so I don't know how that part is now. It seems that the education is more approaching children now, and they have fewer fish in bigger tanks.