Or, What Rock Can I Put in My Saltwater Tank?

In Part 1 of this article, we talked about live rock, and most reef aquarists use at least a little of it in their tank. Spontaneous generation does not exist, and if all of your rock is dead, then it will not magically come alive just by putting it in (artificial) salt water. Sorry about that.

The only way to have live rock is to start with at least a little live rock which will seed the rest of any dead rock--unless you can toss your dead rock into the ocean for a few months or years until you can retrieve it.

Some people say that if you rub live sand on dead rock, you can make it live. And that's possible if your live sand is very alive. However, even if you get your dead rock to become live with the use of live sand, we're talking about bacteria, algae, and perhaps some microfauna. This rock will never have all the good surprises (and occasionally bad) that original live rock has like mushrooms, feather dusters, other worms, and possibly other unusual things. And there's nothing wrong with that because it's the bacteria that you need for filtration. The rest is aesthetic. Your choice.

The more dead rock, and the less live rock you have in your system, the longer it will take for the rock to all become alive, be filled with bacteria, be covered in coralline algae, and become a significant part of your tank's filtration.

The aquarist, however, doesn’t have to use all live rock, which is the most expensive alternative. So, for the sake of discussion, let’s talk about what other kinds of rock you can put in there. And let’s just review for a minute why you have rock in there in the first place:

1. The rock will become colonized with bacteria which will play a major role in your chemical filtration.

2. It looks good.

3. It approaches a natural setting for your tank livestock.

4. It provides ledges on which to grow corals or anemones or zoanthids or clams.

5. It provides hiding places for fish to rest.

6. It provides divisions in the aquarium for livestock territoriality.

7. It provides a framework for microfauna like copepods and amphipods to live in.

So, moving right along, aside from live rock, and “dry live rock” which may be alive or dead, what other kinds of rock can you use?

Dead Rock

I'm reminded of that old comedy skit about Ray J. Johnson Jr.: "You can call me Ray or you can call me Jay, or you can call me Ronnie, or you can call me Johnny...." The same applies here. You can call it base rock, or you can call it Marco rock, or you can call it synthetic rock, or you can call it reef rock, or you can call it Stax rock, or you can call it whatever you want, but the rock in this category is D.E.A.D., dead.

Under the category of dead rock, I’d first put dead coral. By that I mean the calcium carbonate rock that is primarily the skeletal waste of dead coral. Maybe it was washed up on the shore and sat in the sun for 17 years. Or maybe it was quarried inland from ancient reefs. In any case, it’s primarily calcium carbonate, and it’s dead. And it’s pretty porous, which you want. The lighter and more porous your rock is, the more surface area you have for bacteria to occupy.

Marco Rocks is a brand, a company that mines, processes, and sells dead rock. They call it “dry rock.” Perhaps “dry” rock sounds more enticing than “dead” rock. They have, among other things, ancient coral and limestone that they cut up and sell for aquarium use. Since this is mined (quarried) from land, it’s not hurting any ocean reefs. Coral rock is mainly calcium carbonate, and limestone, a sedimentary rock, is primarily aragonite and calcite, which are both different crystal forms of guess what? Calcium carbonate.

Limestone from Yosemite National Park.

Photo is a royalty-free image from freerangestock.com.

Now (almost) everything made of calcium carbonate is safe is the aquarium, as in it’s not going to hurt the water quality. Aquarists used to think that the calcium carbonate rock and sand dissolved over time and added needed calcium to their system, but recent research has shown that that’s not exactly true. Yes, your calcium carbonate rock and sand may very slightly dissolve over time, but they dissolve very little at the pH that we keep our reef tanks. So, you can not depend on this calcium carbonate to “buffer” your tank, and keep the calcium, alkalinity, and pH up where it belongs. If you’re deficient in calcium, then you’ll have to add it another way.

So aside from live rock, dry live rock, dead/dry coral, and dead limestone, what else is there? Well, you can put other kinds of rocks in the aquarium if they won’t dissolve and hurt the water chemistry.

For example, you could put some quartz or granite rock in there. They aren’t totally inert, and may dissolve a very little bit, but they’re reef safe, according to Dr. Randy Holmes-Farley. The disadvantage of them is that they are very heavy. That means they are not porous, so they have very limited surface area available for colonization of beneficial bacteria. But can you use them? Sure. Lose your mind. Reef purists will make fun of you (and probably me for suggesting it,) but it’s your tank. Have at it. These rocks won’t hurt your livestock.

Even if you decide to use something offbeat for your rock in the display tank, you can always benefit from live rock by sticking it in your sump or refugium or any connected tank where it's less visible.

Purple amethyst, a type of quartz.

Photo is a royalty-free image from Pixabay.

Some other mainly inert materials you could put in your tank are glass (as long as it’s not float glass which is manufactured on a layer of usually tin and traces of that metal could hurt your water), fired clay, ceramics, polypropylene, polyethylene, and there are several glues and putties and silicones designed for aquarium use. And if money isn’t an issue, you can make your reef out of solid gold. Go for it. But I digress.

Solid gold.

Photo is a royalty-free image from Pixabay.

Next, there’s also artificial rock. Several companies sell it. I don’t know all the ingredients in the many brands sold, but if it’s designed for marine aquariums, then it’s likely safe. Ask some fellow aquarists their opinions before sinking a lot of money into it. Or ask your neighborhood chemist. I have no opinion on artificial rock.

Last, but not least, let’s talk about DIY rock. Yes, you can make your own rock at home. There are many recipes available for making your own rock as well as plenty of YouTube videos. Making it yourself is dirty, smelly, and it takes time and patience and some brute strength, but you can do it. It’s typically made with some kind of cement, some kind of sand, water, perhaps oyster shells or crushed coral, and some kind of coarse salt. That’s all I’ll say about DIY rock here. I’ll devote more time to this topic in another article.

One thing all these dead rocks have in common is that you still need to cure them, if only for a short while. You need to confirm that they are not leaching anything bad into the water. For example, some types of dead rock are famous for leaching phosphates. DIY rock is always very alkaline and has to be cured until the pH drops to an acceptable level. Since I haven’t explored this curing process (yet) with you for any kind of rock, dead or alive, please be sure to research it and do the curing correctly before putting any rock into your display tank.

Always keep in mind that with an aquarium, you have a closed system. Water quality and stability of water quality is paramount for the survival of your livestock. So, whatever you choose to put in your aquarium, be very sure that it will not hurt your water quality in the short or long term because in a saltwater aquarium you have a very narrow margin of error.


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Author Profile: Cynthia White

Cynthia received her BA in English from NYU a long time ago. She has been a freelance writer and editor for over 20 years. She has written for newspapers and magazines, both in print and online and was formerly a marketing manager for a small oil company. Her portfolio can be found here. Her forum nickname is Seawitch.

For 15 years, she kept a dozen freshwater tanks, bred cichlids--Cyphotilapia frontosa--and sold them to pet stores in Calgary. Finally, after years of study, she has come to saltwater side. She lives in British Columbia, Canada, with her husband and three special-needs dogs, a five-minute walk from the Georgia Strait.