Rock looks nice whether you have a large boulder in your yard or small rocks covering the bottom of your fish tank or aquarium. But when those rocks are exposed to constant moisture, or if you live in an area that has high humidity, this can change your white rocks into a green, slimy looking mess. You can often find algae growing on the north side of a tree. This is because this side of the tree generally stays moist and it is out of direct sunlight. If you want to bring the original color back into your rocks, all you need to do is wash them. If you do this often, you can keep the green algae under control.
Cleaning Aquarium Rocks
If you have small rocks from the bottom of your fish tank, you will need to remove the fish and place them into a separate holding tank. Siphon out most of the water until you are down to the rocks at the bottom. Scoop the rocks out of the tank with your hands or a plastic scoop and drop them into a colander. You may need to do this in batches, if you have a lot of rocks or if they are extra dirty with algae.
Rinse the Rocks Under Water
Hold the colander under running water to wash the algae off the rocks. This may be enough to clean them if there is not much algae growing. If your tank is like mine, I hate to admit but I tend to put things off and don’t clean my rocks until they are totally covered in green. For problems this bad, you will need to continue with the following steps.
Transfer the rocks into a container and fill with water. There is a special tool called an algae scraper and you can find this at most fish supply stores or online. Use this tool to scrub the algae off the rocks.
When you are finished, rinse the rocks under clear running water for several minutes.
Wash With Bleach
Fill a bucket with bleach and water mixture. Use one-fourth cup bleach to one-gallon clear water. Add the rocks and allow them to soak for at least 15 minutes. Bleach will kill the algae and keep it from growing back. Don’t soak the rocks any longer than 15 minutes in the bleach solution. If needed, you can scrub the rocks with a brush, but remember to wear rubber gloves to protect your hands. in the bleach water.
Rinse the Rocks Thoroughly
At the end of 15 minutes, rinse the container out and refill it with cold water. Pour the aquarium rocks in and allow the rocks to soak for another 15 minutes.
Transfer the rocks to a colander again. Hold the colander under clear running water and thoroughly rinse the rocks under the water. You will need to rinse all traces of bleach away otherwise you risk killing your fish and/or aquarium life and plants.
Large Rocks
If you have larger rocks outdoors most of the time spraying them with the sprayer nozzle of you hose will be enough to remove the algae. If you find that this does not clean the rocks to your satisfaction, you can clean the rocks with a nylon brush.
Scrub the Algae Away
Fill a bucket with hot soapy water. Dip the nylon brush into the water, and then scrub the algae off the rocks. If the algae is old, crusted on or just seems impossible to scrub off with the brush, you can use a scalpel or knife to scrape away stubborn algae growth.
Rinse the rocks often with clear water to help wash off the algae as you work. This shows you any areas that need more scrubbing or scraping.
Reference
The Aquatic Critter: Freshwater Aquarium Maintenance
Virginia DEQ ProbMon Benthic Algae Sampling Procedures; Virginia Government 2007
General Glossary for the Biological Monitoring Program; Maine DEP 2007
Family Management: Cleaning and Disinfecting