Undersea City is a free to visit attraction. You can see a lot of trout here. It is very beautiful and the natural scenery is very beautiful.
The Underwater Observatory in Queenstown offers fascinating entertainment for all ages. It shows the underwater inhabitants of Lake Wakatipu including cheeky diving mallard ducks, massive rainbow trout and slinky NZ Longfin eels. Longfin eels are an endangered fish species and the biggest freshwater eels in the world. The Observatory has very big fish and eels enjoying the pellets that you can buy and drop into the water. A nice little outing if you are visiting Queenstown.