DIY Cave & Rock Aquarium Background

Updated November 21, 2016

A cave and rock aquarium background is an easy DIY project that adds decoration to the aquarium and will provide fish with cave spots in which to hide. The background will be three-dimensional and made of concrete or sand covered styrofoam to resemble the look of real rocks and caves. The background will be installed inside of the aquarium with silicone.


Use 50-mm block of styrofoam for the background. Use a permanent marker to draw the rock and cave design and a knife and utility razor to cut and shape the styrofoam to look like rocks. Coat the styrofoam with quick-dry concrete or epoxy resin and sand to resemble rocks, and adhere it to the inside of the aquarium on the back wall with aquarium-grade silicone adhesive.

Design the Rocks and Cave

Measure the inside back wall of the aquarium and cut at least two pieces of styrofoam insulation to that size; cut more pieces for greater depth. Draw rock shapes on one styrofoam piece with the Sharpie, leaving some space to be removed in between rocks. Use an image of cave rocks as inspiration or make up shapes; this should look like a simple drawing of wall rocks. Cut the rocks out of the styrofoam with a razor, then shape the pieces with a knife. They may look more like potatoes than rocks until they are covered with concrete or sand. Use the silicone to glue the rock pieces to the second piece of foam core, creating a rectangle shaped background that will be adhered to the back wall. Glue a third or fourth layer of styrofoam where the cave will be, then carve out the inside. Keep in mind the location of the aquarium filter and leave room for it to fit when designing the rock shapes and gluing them.

Finishing the Background

Take the assembled styrofoam cave and rock background outside or into a well-ventilated area, and cover the floor or ground with plastic dust sheet. Mix the quick-drying cement using the ingredient measurements listed in the product's instructions and add colour pigment or sand if desired. Apply three coats of concrete to the styrofoam background with a paintbrush, letting each layer dry before proceeding with the next coat. Epoxy resin maybe used instead of concrete. Coat the styrofoam with three coats of epoxy and allowing each coat to dry before applying another. Add sand to the third coat of epoxy while it is still wet, and use a paintbrush to brush off loose sand once the epoxy resin is dry.

Installing the Background in the Aquarium

The tank should be empty and clean. Put fish into a holding tank and remove the water from the tank. Lay the aquarium on its back, and coat the back wall with silicone. Insert the styrofoam background into the aquarium and press it against the silicone. Let gravity keep the background in place while the adhesive dries. Fill the tank with tap water to rinse the background once the adhesive is dry, and rinse about 10 more times. Concrete will raise the pH in an aquarium and can take weeks to stabilise. Rinsing with tap water will help speed up the process, but continue to monitor the pH of the tank before you return the fish to the tank.

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