Yes, you could put the completed set of the Ghostbusters Ecto -1 by LEGO inside of an aquarium. This is a wonderful and safe way to display your kid’s hard work. LEGO pieces are safe in aquariums for both the inhabitants and the LEGO pieces themselves. LEGOs are made of durable, hard plastic and it does not consist any toxicity. The pieces will not break down due to being submerged in water. However, it is important to clean the pieces as an entire aquarium should be cleaned regularly to prevent build-up.