I get a lot of questions and comments about the rocks in my aquariums, so I have decided to address most of those questions here in this article. Well, these rocks are called "honeycomb limestone" or Texas "holey rock." And as you may have guessed by its name, it is endemic to TX, which is where I live.
Limestone and dolomite (recalcified limestone) are often called carbonate because they consist primarily of the carbonate minerals calcite (CaCO3) and dolomite (CaMg[CO3]2). Lime-stone is formed by the accumulation of these minerals being deposited on top of each other over a period of thousands of years. A honeycomb appearance may form as the earth moves and fractures form. Seeping rainwater mixes with sulfurous gases in these cracks, which dissolve the limestone. The result is limestone with a honeycomb of chambers.
Because limestone is soluble, meaning it will slowly dissolve (leach) over time, it can be a great addition to any African Rift Lake aquarium. The pH for these lakes are anywhere from 7.2 – 9.0. This is very alkaline for a freshwater aquarium, and almost as alkaline as saltwater setups. The limestone helps, in effect, to raise the pH of soft water by “donating” Calcium, Magnesium, and Carbonate. And if you already have “hard” water, limestone can be helpful in buffering, or keeping your water at a pH no lower than 7.6. Minerals don’t stay suspended in water for very long, and so the limestone helps to keep your pH stable by continuously leaching calcium carbonate into the water. There are lots of rocks besides limestone that will give you this effect, but I prefer honeycomb limestone over others for at least two reasons.
Aesthetically speaking, I find its bone white color very appealing. The limestone along with the crushed coral that I use as substrate (which also helps buffer my hard water) gives the tank a very clean and pristine appearance. The water just seems to sparkle. Some people prefer dark rocks and dark substrate, arguing that their fishes’ colors contrast best with these. Well, I contend that my fishes’ color radiates very nicely with a light-colored background.
The second reason why I prefer it over other rocks is due to the fact that it has numerous caves. Mbuna (i.e., rock-dwelling cichlids) are very territorial. They love caves and little nooks where they can hide from other equally-aggressive tank mates. Some of my Africans spend a considerable amount of time swimming through these tunnels and most of my Mbuna actually sleep in them. Furthermore, I have noticed that by providing them with lots of caves, my dominant fish have claimed less territory.
The most frequent question I get about these rocks is: “Where can I get some rocks like yours?” If you live in Central Texas, it’s all over the place. I have also heard reports that honeycomb limestone also exists in Washington State and Florida. I wouldn't be surprised if it could also be found in many other places. If you want to buy it, there are several places on line, including Ebay.
Before you put these rocks in your tank, be sure to clean them with bleach. And then be sure as heck to rinse that bleach off before you put it in your tank with the fish. The sniff test works for me: if I can smell it, I know it’s still got bleach on it. I have purchased limestone that was purportedly "clean." When I got to cleaning it, I found all kinds of dirt and roots in some of the holes of my limestone. I was glad that I took the extra precaution. In fact, I pulled a rock out to clean it after 6 months and found some dirt in holes that I had missed when I initially cleaned it.
Because of the immense weight of your rocks, especially if they are stacked, you should consider how they are placed in your tank. This is especially important because your African Cichlids will dig and could undermine a rock formation that could come tumbling down. To prevent such a disaster, I glue rock formations together with silicone. Some people use egg crates on the very bottom of their tanks, before laying gravel. This prevents a rock from applying pressure to a single point on the glass, which could lead to crack.
If you’re unable to find any honeycomb limestone, there are other Cichlid-friendly rocks I recommend. These include slate, quartz, petrified wood, lava, granite, tufa, "pagoda," and "lace rock." □