On the Use of Crushed Lava Rock
By Joseph Gardner
1998, Issue 14
Crushed red of gray lava rock has several good uses in the aquarium. It is easily obtained and attractive. Crushed lava rock such as comes in fifty pound bags at places like Wal-Mart and K-Mart and garden supply shops is a good thing to use in aquariums for several reasons. Crushed lava rock is about one and one-half inches long and does not seem to impact water conditions as much as crushed marble would, though marble would probably be good to use in an African cichlid tank. It is a fairly inexpensive medium, one fifty-pound bag went a long way. I used it to cover the bottoms of the following sized aquariums: a twenty-nine gallon , a twenty, a forty breeder, a ten and a five gallon.
For fish which like to rearrange the gravel, crushed lava rock is not as easily moved as gravel. In my cichlid tanks, the fish have not been able to do much rearranging. Somewhat surprisingly, the red lava rock has not been a factor in changing any finnage. I thought with as rough as the rock is, it very well might. In addition, the use of lava rock can help protect potted plants from being dug up in both South American and mbuna tanks. I also use the lava rock as a filter medium. I use sponge filters and then cover them with the rock. In this way, I believe the lava rock with all of its pores, helps to clean the water. The lava rock, because of the pores, has a lot of surface area for bacteria to grow in. The above mentioned tanks, even though crowded, are crystal clear. The lava rock does not discolor the water, for those who are concerned about such things.
An additional use is as a way of protecting fry. When I use crushed lava rock in my tanks, I know that when I go to remove the rock, I will almost always find young fry. When I was trying to catch a mouth-brooding female L trewavasae , something moved at the bottom of the tank. It turned out to be a L. zebroides fry, almost 2 inches long. It was able to hide in the rock and survive with full-grown L.trewavasae, a very nasty combtail, and other large Africans. I also use lava rock in rearing tanks. A layer about two inches thick will provide many cracks, crevices, and small caves for the fry. Smaller or weaker fry can hide from the larger or stronger without too much damage or stress.
Preparing the lava rock is fairly simple. I usually run water over the rock until it runs clear. Some caustion is needed when washing the lava rock because all the sharp edges can scratch you. Then place the rock in the aquarium in a layer thick enough for your purpose. I have never used lava rock with an undergravel filter, so I do not know how that would work, but I doubt it would be less effective unless the lava rock was crushed up even further. Lava rock seems to work well with sponge filters, which I bury, canister filters, power heads mounted on sponge filters and power filters. You will need to be careful with the lava rock, it will scratch acrylic and glass aquariums. I have found lava rock to be a good and versatile substance to use with aquariums. I hope you will give it a try. The end