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 Rocks In Aquaria: A Geologist's View By Laetacara1 
Rocks And Aquaria: A Geologist's View
Rocks And Aquaria: A Geologist's View

By John Wakabayashi


Copyright, March 2003

Periodically there are posts on the subject of rocks in the aquarium. I think as a geologist (for more info see my website) , I need to clear up some things about the use of rocks in the aquarium: what to be aware of for their use in the aquarium, and ways of testing them.

Rocks appropriate for the aquarium can be found nearly everywhere, whether it be by a stream, on a hillside, or on a beach, or in your backyard. Rocks innappropriate for an aquarium can be found nearly everywhere as well.

Many rocks do not dissolve rapidly enough on human time scales to have any impact on aquarium chemistry, but some do. Rocks to be avoided for any aquarium set up those with metallic minerals. Some metallic minerals can react with water to produce sulfuric acid (because many are sulfides), lowering the pH as well as releasing some potentially harmful metals into solution (copper, lead, arsenic occur in many). Metallic minerals are best identified on the basis of their metallic appearance. Some shales and slates have disseminated pyrite (iron sulfide: FeS2) that is too fine to be seen without a microscope. I once studied a site where the groundwater was acidified because it ran through a formation of shale that contained a bunch of microscopic pyrite.

For softwater, neutral-acidic aquariums, any rock with carbonate minerals should be avoided. Carbonates will dissolve, harden the water and raise the pH. Carbonate minerals occur in carbonate rocks, made up primarily carbonate minerals (limestone, dolomite, marble), coral and shells (composed almost entirely of calcium carbonate), veins in many sedimentary and metamorphic rocks, and cements (usually too fine to be seen with the naked eye) between grains in many sandstones, siltstones and shales.

Now, for the carbonate test: Carbonates will react and give off CO2 when in contact with hydrochloric acid. What about vinegar? I often see posts about this. Vinegar, and other weak acids will NOT detect many carbonates. Vinegar is a very weak acid. Even concentrated acetic acid (vinegar is dilute acetic acid) is orders of magnitude weaker (in hydrogen ion activity) than even the comparatively dilute 1 molar HCl solution most geologists carry around. Carbonates such as dolomite (a calcium-magnesium carbonate) bubble only weakly even in 1 M HCL; they won't do squat in vinegar. Even calcite, the most common carbonate (CaCO3), if disseminated as cement in some sandstones may not give an explosively effervescent reaction. So even with HCl, you have to watch carefully. I realize that HCl isn't available at your local supermarket, but, then again, neither is Amquel, Biospheres, or any of the fishy things we usually put in our aquarium. HCl is easily available at a scientific supply store. I was a rockhound as a kid, and I always had HCl obtained from such a store; such stores are no more uncommon than an LFS. A little bit of HCl goes a long way; 50 ml is all you need to test rocks to your heart's content. HCl is also available (for cheaper unit price) under the trade name Muriatic acid (3M HCl last I checked) at pool supply stores, but you'll have to buy a larger amount. It is usually available at pool supply stores. pH Down is another commercial product that may work for most carbonate-bearing rocks.

Some other type of rocks that should be avoided for soft water/neutral-acid set ups are evaporites, the most common of which is gypsum (a hydrated calcium sulphate)--this will harden your water with Ca++ and S04 (2-). Beer brewers (Judy and I are) commonly use gypsum to harden their water (hard water brings out hop bitterness more). Adding gypsum to brewing water is attempt by brewers to duplicate the natural brewing waters of Burton-on-Trent in England where water is very high in calcium and sulphate, presumably because of gypsum in the rocks of the drainage basin. Gypsum crystals are clear and can be scratched with your fingernail. They may be a bit harder to identify when they are cements in sandstones and shale and fracture coatings; these are common, more subtle forms of gypsum. As a cement gypsum is difficult to identify because although it is soluble in HCl, it does not effervesce (because no gas is released during the reaction), so you really won't know it's dissolving.

So, what does this all say about rocks in aquarium?
1. Avoid all rocks with visible metallic minerals.
2. If you are running a soft water/neutral-acid tank, avoid rocks with carbonates and gypsum.
3. Test for carbonate minerals with 1 M hydrochloric acid or equivalent.
4. If you are creating a hard water set up for Central American or African Rift Lake cichlids, carbonate bearing rocks may be OK, but you should monitor your pH and hardness to make sure it does not rise above acceptable levels. In some cases, carbonate-bearing rocks may help an aquarist with soft water who wants to run a hard water set up. Ideally, rocks and substrate used in this fashion should be added to a tank before the fish are so that pH and hardness can be monitored.
5. Many slates and shales will be OK, but many have disseminated pyrite that will acidify your water, perhaps more so than you'd want even for a softwater/low pH set up. Avoid these unless you know of a geologist who will look at microscopic thin sections of these rocks for you.


Now, note that I said gypsum is pretty hard to pick out as a rock cement. This means the following:

For softwater setups, avoid sandstones, siltstones, and shales, because you will be unable to test for gypsum cements (unless you know a geologist who has looked at thin sections of the rock under a microscope, or a geologist has told you that the particular geologic unit of interest does not contain gypsum). In California, we have many sandstones, silstones, and shales that have gypsum cement. There are sandstones, siltstones and shales from certain parts of the world that are composed nearly entirely of quartz, with a subordinate amount of feldspar (called "quartz arenites) that usually have only quartz cement and will be OK in an aquarium. Again, however, unless you are a geologist or know one, I'd stay clear of sandstones, silstones and shales.

In the absence of tests, granitic rocks are essentially always safe (as long as you are confident identifying them): These include granite, granodiorite, monzonite, diorite, gabbro, and others. Bear in mind that some sandstones can look superficially like granites; even geologists miss this one sometimes.

Most volcanic rocks and their metamorphic equivalents are safe: basalt, andesite, rhyolite, etc. There are rare carbonate-bearing lavas, but these are indeed rare. Volcanic rocks originally deposited underwater sometimes have filled voids or lenses of carbonate, so volcanic rocks of all sorts should be tested with acid. Metamorphosed volcanic rocks are commonly greenish in color (if metabasalts or meta-andesites) and are generally safe, except for the occasional carbonate vein.

Do we (John and Judy) have rocks taken from outside in our aquariums? Yes. We have several rocks that I've collected in the field in our tanks. They include blueschist and amphibolite (metamorphosed basalt), quartz diorite (a granitic rock), and granodiorite (another granitic rock). None of these rocks were acid tested for carbonate, only because I've looked at thin sections of these rocks under the microscope and know that there's no carbonate in them.


Copyright 2003, Aqua Den. ALL RIGHTS RESERVED.


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