I think there are some misapprehensions here. In the first place the article mentions the figure of a thousand fish: that's a cost of a few thousand pounds straight away (assuming that they aren't all guppies and zebra danios). Then there is planting, heating, lighting and filtration equipment, plus the cost of construction of tanks and at least one pool (shown in the video). An aviary for songbirds, on the other hand, just needs a frame and some mesh.
The main cost is obviously the building itself. By comparison with a simple family home, all zoo buildings seem eye-wateringly expensive. There are several reasons for this: they are all one-off designs, which have to meet the needs of the animals, the staff and the public. They need custom-built heating, lighting, ventilation and waste management systems. They also need to be attractive and welcoming to encourage visitors to come to the zoo. I expect that one of the factors behind the decision to build this exhibit is the zoo's need to provide somewhere warm and dry for visitors during the winter and in bad weather at other times of year.
I look forward to reading reports from ZooChatters when it is actually open.