Place a large glass container or open tank shape using blocks and glass walls
Build a watertight frame with blocks around the edges
Line the inside with glass walls or clear blocks
Fill the tank with water using buckets or a water source
Add sand, stone, coral, plants, or other underwater decorations
Place fish, sea creatures, or decorative critters inside if available
Add lighting with torches, lanterns, or glow items outside the tank
Seal any gaps so the water stays contained
Expand the aquarium with multiple sections if desired
Use platforms, doors, or pumps to create viewing and access areas