Aquariums can be made of quite literally any material provided it will allow you to hold sufficient water and do not adversely effect the chemical balance of the water.

Thus concrete aquariums may be a choice for a garden or outdoor aquarium, perhaps set into the ground like a pond style. Remembering that concrete will by nature make the vessel a lot heavier and not perceptible through normal side viewing only granting a top-down view.

Concrete is generally the preferred medium used for larger, commercial projects or concrete aquariums such as the Tennessee aquarium or Charleston aquarium. The only potential problem is that sometimes cracks appear in concrete which will allow water to seep out. Such fractures in the structures can be repaired in-part with silicon fillers.