DIY Rocks for a Freshwater Aquarium

Rocks look great in freshwater aquariums and give fish a place to hide when stressed. But do not throw just any old rock into the tank. Rocks may be covered with pesticides, herbicides or other chemicals that may poison the fish. Rocks slowly dissolve in water, so the fish keeper needs to be sure that these dissolved chemicals will not kill the fish.

  1. Choosing Rock Type

    • According to "Tropical Fishlopaedia" (Mary Bailey and Peter Burgess; 2000), even rocks listed as being safe for freshwater aquariums may not be. Avoid all coral (technically not a rock, but sold as a rock) because it will leech dangerous chemicals into a freshwater aquarium. Avoid all easily crumbled rocks and rocks with metallic-looking veins. Stick with gneiss, granite, limestone, sandstone and types of slate that do not have any metallic or gold-colored veins. "Freshwater Aquariums for Dummies" (Maddy and Mic Hargrove; 2006) states that the best way to test if a rock is safe enough for a freshwater tank is to spoon a couple drops of vinegar onto it. If the vinegar begins to bubble, it is already leaking chemicals that can kill fish. Do not place these rocks in a freshwater aquarium.

    Other Considerations

    • "Freshwater Aquariums for Dummies" states that fish prefer neutral colors like grey, brown or tan because these are colors normally found in the bottom of rivers, streams and creeks. They do not like very bright colors. This is probably because fish do not want to contrast so sharply to their surroundings that they become an immediate target for predators.

      Never paint a rock intended for the aquarium because the paint could kill the fish. Make sure there aren̵7;t any sharp edges on the rocks that a fish can hurt itself on. That could happen easily when startled fish hurl themselves around the aquarium.

    Preparation

    • Scrub the rocks with a new plastic or wire brush in plain water to remove any dirt, insects or moss. Use only a brush that will be used to clean aquarium decorations and has never been cleaned with soap. Never use soap or dish detergent because that will never completely rinse off of the rock and the soap traces will poison the fish. Place the rocks in a new bucket or one that has never had soap or detergent in it. Pour boiling water over the rocks and leave them to soak overnight before putting them in an aquarium.

    Warning

    • "Tropical Fishlopaedia" warns that rocks, pebbles or gravel should never be taken from natural habitats like a river or streambed. These rocks provide essential places for native freshwater fish species to spawn. If the rocks aren̵7;t there, the eggs may not have a place to stick to or the baby fish (fry) will not have a place to hide as they grow. It also may be illegal to take any rocks from private property or even parks and nature reserves.