Homemade Conglomerate Rocks

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After many years in the aquarium hobby I found that I accumulated a number of small pieces of coral rock that were either too small or not the right shape to use in my reef. Instead of discarding all of this material, it is always better to recycle. These days, reef rock can run up to $7.00 a pound, and a pound of rock is not a very large piece of rock. Very nice looking rock can be made using these fragments and a little cement. The best part is that any size or shape rock can be made for very little cash.

To cement these pieces together I use Sakrete mortar mix. I have used this for over twenty years with no problems. Start with the larger rock pieces for the basic shape and size of the rock you are trying to build. Lay out all fragments of rock you have to work with. If the pieces have algae stuck to them, then you should soak them in regular bleach and water for a day. Use only "regular" bleach and not scented, this is very important. Try to picture the finished rock. Mix a small amount of the mortar mix according to the directions on the bag. Wet the rocks in fresh water before you cement them together. Smear the mortar mix onto the edge of one of the large rocks, pushing the mix into the natural holes in the rock. Now, take another large rock and do the same thing. Place the rocks together and build up the area that touches with the mortar. You may have to prop up the rocks temporarily with other rocks until the cement dries. You can put as many rocks together as long as you have them propped up so they make contact. Do not try to put too many rocks together the first day. If this conglomeration falls, you will have to place it together again. Let the cement cure for at least a day by keeping it damp. I put plastic wrap over it. After it's cured, mix some more cement, gently pick up the cured cemented rock and fill in the original cemented joints with the cement. do not try to make the joints smooth, try to mimic the rock that you are cementing. Before the cement cures, take a toothpick and push holes in the joints to approximate the holes in the original rock. You don't have to be an artist. Cure this for a day or two the same way.

After you have the "backbone" built this way, you can start adding smaller pieces of rock. Try to have the pieces stick out from the "backbone rock" Fill in the joints the same way. You can put bricks next to the rocks to support them while the cement cures. Mount the smaller pieces of rock with the broken edges cemented to the backbone rock and the rounded sides facing out. The more bizarre, the better. Design mounting holes into this homemade rock to hold pieces of live coral.

The beauty or this is that you can custom build your reef and build it to hide filters and heaters. You can make platforms to hold corals, and design large holes for good circulation. I like to design the conglomerate rocks about 18" long. To buy a rock this size would be very expensive and you would not be able to find one with the exact specifications that you want.

Cure the finished rock for a few weeks in fresh water. Change the water every few days. These conglomerate rocks have very little cement in them and they will cure quicker than a total cement rock. Still, it is safest to test the cured cement for high PH after you think it is cured to make sure that it is not raising the pH of the latest change of curing water. After a few weeks in a reef tank, you will not be able to tell which rocks are natural and which are homemade. In my reef, which is about twenty five years old, even I can't tell which rocks I collected and which ones I made.