How to Make Natural Stone Fish Tank Backgrounds

Choose stone that will not harm the health of your fish. The stone should not affect the pH of the water inside the tank. It should also not release any harmful minerals into the tank water. Stones that meet these requirements include volcanic rock, slate and granite.

Inspect each stone and remove any that you find with sharp edges or rust spots. The sharp edges can harm the fish, and the rust is a sign of a mineral change in the rock.

Wash and dry each stone. Take the bottle of hydrochloric acid solution from an aquarium nitrate test kit and put a few drops of the hydrochloric acid on each stone. If, within a minute or two, you begin to see foam, fizz or bubble, discard the stone. Stones that foam, fizz or bubble will release calcium carbonate into your aquarium water.

Scrub each stone with hot water and a hard brush and place them in a bucket of warm water for one day. Do not use bleach on the stones as it is hard to completely remove. If a stone begins to crumble or flake during this cleaning process, do not use it.

Place the larger stones on the bottom back wall of the fish tank to act as a support.

Continue stacking the stones upwards along the back wall creating small "caves" as you go. Check each stone to ensure that it is well seated and will not fall out of place. You can fill in any unwanted holes and create more stability with smaller rocks or gravel.