Spent the day at the Berlin zoo and bought the zoo/aquarium combo tickets. Berlin welcome card gives both a 25% discount.
The zoo is pretty and nicely laid out. Going on a wed also means there is not a huge crowd. Our two-year old really enjoyed it.
In hindsight, we probably shouldn’t have got the ticket to aquarium - it’s not one of the most impressive aquariums. Plus the zoo itself can easily take 4-5 hours to explore, especially if you visit with young babies or want to explore at a leisure space.