We went here on a whim - it wasn't originally on the list we had for Vienna, but I'm very glad we went. We expected just an aquarium, but the range of exhibits here is much larger than you might expect. There's a multi-storey open area with monkeys, tortoises and a crocodile, a few open areas with birds, reptiles, snakes & spiders, and the expected large tanks with fish & sharks. Pleasantly surprised with the range of exhibits.
There is also a restaurant / café on the 11th floor, surrounded by a viewing platform for views over the city. The viewing platform is open to access, either as a café visitor or an aquarium guest. Great photo opportunity.
The venue recommends for the aquarium starting at the top floor and working down, though if I were to go again I would start at the bottom and work up. The flow through the aquarium seems to work better that way.