Homemade Live Rock from Cement & Oyster Shells and cycling your new reef tank!

You can make your own live rocks with cement (CEM II type with limestone) and crushed oyster shells so that you can make the reef tank aquascape of your deams! By making your own, you can produce them in any shape or size, all exactly as you like - and at the same time limit your effect on wild reefs. Aquacultured live rock is another good option, where quarried dry rock (like MarcoRocks) is left in the ocean for several months and then sold.

What's CEM II Portland cement? Good question! I wasn't sure either - but it just means it's got up to 35% Limestone (or other things) in it. You want cement made with limestone. CEM I is just pure Portland cement. I'm sure it would probably work, but the additional limestone in the CEM II cement will likely help moderate your tank's pH a bit better (though it shouldn't leach much at all really once dry).

So what are the directions for making your own live rock from cement & oyster shell? Watch the video for all the details, but it's really not too hard. You can use whatever size crushed shells that you like visually, as they all worked in the experiments. The smaller the shell, the more dense your rock will be. You can get various size crushed oyster shell in bulk easily - it's used for all sorts of things including Bocce Ball courts.

Use a ratio in grams of 1 to 3.1 cement to crushed oyster shell. Figure out those amounts, and then use 50% of the weight of cement in water. The basic directions are...

- Rinse the shells to remove any dust
- Bleach the crushed shells with unscented household bleach for several hours
- Rinse them well in fresh water
- Maybe repeat the bleach and rinse process just in case
- Dry them completely (in the sun, or in an oven as in the paper)
- Mix the shells and water, they should be just wet
- Mix in the cement
- Form it into whatever shape you like using your hands or molds made from styrofoam
- Let it dry following the directions for the cement.

You can find the paper here with all the details and test results:
Relieving pressure from coral reefs: Artificial oyster rocks can replace reef rocks used for biological filtration in marine aquariums
Felipe P.A. Cohen, Ana Elisa Cabral, Ana Isabel Lillebø, Ricardo Calado
https://doi.org/10.1016/j.jclepro.2021.129326

Abstract
Natural reef rocks have been harvested from the wild for decades to supply the marine aquarium trade. These rocks have been used in home aquariums mostly as substrate to safeguard biological filtration, especially for nitrification. Producing artificial rocks can be a sustainable alternative to the harvest of natural reef rocks from the wild, thus promoting coral reefs conservation. Nonetheless, there is a lack of scientific knowledge on standard formula to prepare artificial rocks, as well as on their potential to promote nitrification. The use of oyster shells, a by-product from oysters’ aquaculture, appears as a promising ecological and economic approach to produce artificial live rocks. Thus, we aimed to develop a standard formula to produce artificial rocks made with oyster shells and test its potential use as a substrate for biological filtration in marine aquariums. For that, we prepared artificial oyster rocks with different sized fragments of oyster shells and compared their ability to promote nitrification of ammonium nitrogen with that of natural reef rocks and fragmented oyster shells. The formula was set at 1 g of dried cement (white Portland cement with limestone) per 3.1 g of dried oyster shell fragments. The volume of water used to prepare this mix was set as 50% of the cement weigh. Our results indicate that artificial rocks made with oyster shells and cement display an ability to promote nitrification similar to that of natural reef rocks, promoting a slight decrease in pH and without releasing PO4–P. All treatments with substrate were able to oxidize up to 21 mg.L−1 of TAN (total ammonia nitrogen, initially added in the form of NH4Cl) within a 48-h interval. In other words, it is possible to safely assume that all tested substrates oxidized an average of ∼4.4 ± 0.1 μg of TAN.g−1.h−1. Thus, our results support that artificial rocks made with white Portland cement and oyster shell fragments are a sustainable option to safeguard a suitable biological filtration in marine aquariums.

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