Things You'll Need
- Styrofoam
- Silicone sealant
- Knives
- Heat gun or hair dryer
- Cement mix
- Cement acrylic fortifier
- Paintbrush
- Tint
Instructions
Determine the size of the cave that can fit into your fish tank. Styrofoam is available in many sizes. If you purchase sheets that are too thin for your needs, use a silicone sealant approved for aquariums to glue two pieces together.
Shape the outside portion of the Styrofoam with a knife. Remove all straight edges and corners from the piece of foam, but do not be overly concerned about creating rounded corners at this time.
Remove pieces of Styrofoam with a knife to create the interior sections of the cave.
Heat the Styrofoam with a heat gun or hair dryer to remove any remaining rough edges on the cave. The heat will melt the foam and create rounded sections. Do not overheat or you'll risk burning the Styrofoam.
Combine cement mix and concrete acrylic fortifier with water according to the manufacturer's directions. Add enough water so that the mixture is runny.
Paint the runny cement mixture onto the Styrofoam cave with a paintbrush. Allow the cement to cure for 24 hours. This thin layer of concrete will provide a bonding agent for the next layer.
Mix another batch of concrete and concrete acrylic fortifier, but use less water so that the consistency of the mixture will hold a shape.
Apply concrete mixture to the Styrofoam cave with your hands. Allow the concrete to cure for 48 hours.
Make another batch of runny concrete to add to the cave. This final layer is the finishing touch. Add concrete tint to the concrete if you want to change the color of the cave. Adjust the thickness of the mixture if needed to achieve a smooth finish on the cave. Apply the final concrete layer with a paintbrush and allow to cure for 24 hours.
Apply silicone to the bottom of the Styrofoam cave, along with any other sections of the cave that will come in contact with the sides of the aquarium. The Styrofoam, even with the concrete, is still buoyant and will float if not glued down properly. Allow silicone to cure completely, typically around 48 hours, prior to adding water to the fish tank.