Homemade aquarium decorations can range from simple items of nature, such as collected rocks, driftwood or shells, to elaborate cast or sculpted caves and coral reefs. The biggest concern in making or collecting items for an aquarium is to ensure they do not harm the fish.
There are fish-safe clays and materials, inert once cured, that can be sculpted or molded into endless shapes and colors. Small jars, bottles and unglazed clay pots also provide needed hiding places. All fish benefit from the addition of aquatic plants, which add beauty, hiding places, dissolved oxygen and nutrients to the water. They also help to filter fish waste.
It is important to use caution before adding found objects to an aquarium. Driftwood or shells from a beach may contain enough salt to be harmful, whereas those collected from a river or stream may harbor unseen parasites. In either case, it helps to soak the items in fresh, filtered water, with the addition of enough food-grade hydrogen peroxide to cause slight bubbling on the surface. This kills any parasites, yet leaves no toxic residue. The items must be rinsed thoroughly in filtered water, preferably overnight, before adding them to the aquarium.
Some rocks are also harmful to aquarium fish. These include large salt crystals or certain rocks that can alter the pH of the water, such as limestone. Granite, marble and jasper are relatively inert, and are typically safe to use with fish.