|Cost to Build||₡20,000|
The Aquarium is a level 5 unique building in Cities: Skylines. To unlock it, you’ll need to have 5,000 children studying in your elementary schools (all at once, not aggregate). And you’ll also need to have already reached the Big Town population milestone, which can be anywhere from 1,800 to 7,500 citizens.
You can check your progress towards this unlock by hovering over the Aquarium’s silhouette in the level 5 tab of the Unique Buildings menu.
Once you meet those two requirements, you’ll be able to build the Aquarium for ₡20,000. It provides 150 entertainment value, which is fairly high considering its cheap price, if you compare it with other unique buildings. It also produces only 40 noise pollution.
Note: the aquarium is a requirement for the Hadron Collider monument.
Building Elementary Schools
The Elementary School can be built as early as the Little Hamlet milestone. It costs ₡10,000.
If you have the Green Cities DLC, you can also build the Community School, which serves the same purpose.
It’s generally a good idea to build an elementary school as soon as you’re able to, since having educational facilities can encourage more people to move into your city.
Each Elementary School can accommodate 300 students, while each Community School can accommodate 250.
Having 5,000 Elementary School Students
While your task for this unlock is pretty simple—making sure you have enough elementary school capacity—expect to have a fairly sizable population by the time you can get 5,000 elementary students at the same time.
The bulk of your population is always going to comprise adults (from young adults to seniors), so expect to only have a small fraction of your population going to elementary.
While you can’t really change the demographics of your city, you can slightly boost the birth rate by building a Child Health Center.
You can build the Child Health Center from the Healthcare build menu for ₡18,000.
Other than that, just keep growing your city and making sure you’re building enough elementary schools and community schools as you go along.