This project outlines how to create a decorative DIY aquarium cave out of PVC pipe and some extra substrate or sand. The best thing about this project is you can make caves that match your existing substrate exactly for the most realistic looking cave.
Step 1: Begin by cutting the PVC pipe to the desired overall cave length. A length of around 5 inches is good for smaller caves but you can make the PVC aquarium cave as long as you would like.
Step 2: Cut the pipe from one end to the other as shown in the picture below. The pipe should not be cut exactly in half, but the bottom 25% to 35% should be cut off. This will give you some extra room to push the cave down into the substrate a little and still retain enough height for the fish.
Step 3: Cut the ends of the remaining top portion of the pipe to form a trapezoid when viewed from the side. This will allow easier entry and exit for the fish and give the cave a more natural appearance.
Step 4: Sand down the remaining pipe to ensure all of the edges are smooth. Also all surfaces of the pipe should be sanded to assist in bonding the adhesive to the pipe.
Step 5: Use the adhesive to glue the sand, rocks or substrate to the sanded PVC pipe top. Depending on the grain of the substrate you may want to do multiple coats.
Step 6: If using silicone, allow it to cure at least 48 hours. Hot glue needs to cure for about an hour.
Step 7: Soak the completed cave in tap water for a day then rinse thoroughly.
Here is a photo of the completed project