This guide will let you know how to make an Aquarium in Bloxburg.
The first thing you will need to do is make a 4*2 size pool. Then press the little house icon on the right side, and you will be able to place the basement without colliding with the pool.
Extend the basement a bit and place two walls next to the pool. Then place a window on the wall.
After that, you will need to search the aquarium picture and place it on the back wall. Search two smaller ones and place them on the side walls.
You have made the aquarium now; decorate it the way you want.