Fish tank decorations are fun, but knowing what’s safe to add is important. The standard fish tank decorations you can buy are obviously fine. Aside from plants, other types of decoration include wood and rocks. These may need some preparation before hand, just to be on the safe side.
Rocks such as your standard boulders and pebbles are fine as fish tank decorations, but some rocks will alter your water chemistry. Avoid chalky rocks, these are often used in marine style tanks and can drastically change your water parameters. Sharp rocks can also be a problem as some fish can injure themselves. A simple test can be done by pouring white vinegar over your rocks to ensure that nothing bubbles or fizzes. If it does, then that means the rock is leeching some sort of gas. However if this doesn’t happen, a quick scrub to clean the rocks is all that needs to be done.
Wood needs to be dead, and dried out like driftwood, or bogwood. It needs to be a specific type to ensure it will be safe. Wood that isn’t dead can leech sap and other substances, which isn’t good for an aquarium. Wood from an aquarium shop is usually fine. When sourcing yourself, it’s important to do research before hand. To make sure it’s safe to add to the aquarium, (even from an aquarium shop) some people actually boil the wood. While pouring boiling water over it from a kettle and leaving it to soak is also fine.
Leaving wood to soak in a bucket will also help it to sink once put in the tank, as wood is buoyant and will float. Leaving it to soak will also remove all the tannings. Wood will leech tannings and stain water a tea colour. Some people purposely do this, as the tannings give a more natural look, and can be beneficial to some fish.
Caves are great fish tank decorations to add the tank, as these can also provide homes. Certain species of fish like catfish will actually claim these homes for themselves. Some species of fish will actually lay eggs in these caves, and guard the entrance. A standard ceramic plant pot submerged in the substrate can be efficient, as well as a coconut shell. Avoid any decorations with sharp edges, or use a file to smooth them out. Caves can also be made from slate.