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Suitability of Rocks in the Aquarium - A Practical Guide
by Bob Wyllie

Most rocks that you find in your yard, in creeks or streams, or elsewhere in nature pose no problems for use in the aquarium. Those few that may cause problems can be identified by applying a few simple procedures or observations.

First, the rock must be water-resistant. If it has a tendency to crumble or flake underwater then it will make a mess in your tank eventually. This is easily ascertained. Hose the rock down while scrubbing it with a stiff brush to dislodge all loose surface material. Put it in a bucket of water and leave overnight. The next day give it another vigorous scrubbing. If it continues to shed a significant amount of particles, it is probably unsuitable for your aquarium.

Shale, the softer relative of slate, can sometimes present an unusual problem. Some shales contain a high percentage of organic matter. Even if they are hard enough not to crumble underwater, such shales can release hydrocarbons (petroleum) into water which will collect as an oily scum on the surface. Clearly, you do not want oil-shale in your tank. Most shale does not present this problem, but it is worth bearing in mind when you inspect water that rocks have been soaking in.

Besides being water-resistant, we must be wary of rock that may alter the water chemistry in an undesirable manner.

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Can I use limestone rock?

Perhaps the most common concern is the effect that carbonate rocks (limestone, coral, calcareous sandstone, Texas Holey Rock, etc.) may have by raising the pH and hardness of the tank water. If you have an African rift lake cichlid tank this is clearly a benefit, rather than a concern.

In practice, the power of limestone rocks to significantly raise the pH in an aquarium appears to be rather limited. This can be attested by many African rift lake cichlid aquarists who have found that simply having limestone rocks in their tank will not buffer the water up to the levels they may desire.

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This means that if you have a tank running at pH 7 with any of the myriad fish that are happy at a near neutral pH such as tetras, corys, many South American cichlids, kribs, gouramis, rasboras, danios, etc etc. then carbonate rocks are not an automatic no-no, as is often suggested. In most cases, such rocks may cause the pH to drift up a few decimal points but it will normally remain within the acceptable range for such fish. You can easily check this by putting such rocks in your tank and testing the pH at weekly intervals. If you find it is moving up steadily and reaching undesirable levels for the species, just remove it. It will not have harmed your fish in the meantime.

On the other hand, if you run a tank with acidic soft-water conditions at pH 6.5 or below for certain species, then you clearly do not want to introduce anything that will tend to move the pH or hardness upwards. In such cases all carbonate rocks should be strictly avoided.

How do you identify carbonate rocks? The short answer is the acid test. It is often suggested that dropping vinegar on the rock and seeing if it fizzes will identify a carbonate. This works in some cases, but not all. Vinegar is too weak an acid to provide a reliable test for carbonate. You should use dilute hydrochloric acid, like field geologists do, to determine carbonate. Dilute HCl., sometimes called Muriatic acid in hardware or pool-supply stores is an infallible indicator of carbonate rocks. Put a drop or two on the rock and if it has carbonate content, it will fizz like soda pop. Dilute HCl is corrosive so you should wear gloves and eye protection as you would do when working with ammonia, bleach, or other such chemicals.

What about heavy metals?

Apart from the effects of carbonate rocks on pH and hardness, the other main concern about rocks is-will they introduce heavy-metal pollutants into the tank?

The short answer here is that it is very unlikely. However, there are certain basic precautions to be observed.

Never collect rocks for your tank from old mine dumps or from watercourses immediately downstream from such dumps or old mines. You can often find pretty mineral specimens in such places, but they are not for your tank.

Heavy metals (lead, zinc, copper, cadmium, etc etc and even iron) are toxic for fish, inverts, and mammals, including us, when they reach certain-enormously varying-concentrations. In the case of fish, such metals are primarily dangerous in their free ionic form in the water where they are available for uptake by the fish metabolism.

In the form of compounds, such as oxides, they are inert and not free for metabolic uptake. For example water conditioners which 'detoxify' heavy metals actually bind them into compounds which cannot be absorbed by the fish

In water of neutral or alkaline pH heavy metals normally remain locked into such compounds and remain inert.

Acidic water (pH less than 7) on the other hand activates these metals and renders them potentially dangerous.

Iron, the commonest of all metals, is normally present in nature in the form if iron oxide, i.e. rust. It follows that if you have rocks containing iron, betrayed by rusty streaks and patches, it will not normally present a chemical problem for your fish since the rust, iron oxide, is inert, However, if there is enough rusty material it will discolor your water, so rocks that are heavily 'rusted' should not be used. A few patches or spots of rust here and there on rocks is not a problem.

If you consider that the rotor of every pump motor in your tank is a cylinder of pure ferrite (magnetic iron powder sintered with strontium, barium, etc.) rotating at high speed on a steel spindle in constant contact with the water and at a temperature above the tank ambient (i.e. perfect conditions for chemical reactions) you can see that there is little need to worry about a few rust stains on rocks.

You may have heard about the problem of iron toxicity for wild fish, particularly in northern North America and Scandinavia. Without going into too much detail, the problem is mainly related to the activation of iron toxicity by low pH water during Spring snow melt and runoff, conditions you will not have in your tank-but see below.

Unless you collect rocks from around old or current mining operations you are unlikely to find specimens that contain significant amounts of any other heavy metals. If you do, consider staking a claim!

Beware of fool's gold

There is one important exception-iron pyrite or fool's gold. This mineral is iron sulphide (FeS2) and can be found in rocks as varying as limestones, sandstones, shales/slates, or quartz. It is common in mine residues but it can be found almost anywhere.

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The potential dangers of pyrite are threefold:

  1. It has a strong acidifying effect, i.e. it drops the pH.
  2. In addition to iron it sometimes contains small quantities of other more dangerous heavy metals such as lead, zinc, and copper.
  3. Because it acidifies the water it can 'unlock' heavy metals into their toxic free-ionic forms.

The chemistry is as follows. Oxygen (and natural bacteria) in the water oxidize the sulphide component of pyrite to sulphate (i.e. sulphuric acid). The low pH acidic water produced then activates the iron and any other heavy metals present. Thus it is a triple whammy.

Pyrite is the active agent in the notorious acid-mine-drainage (AMD). AMD is just as likely to arise from coal mines as from metal mines. This is because pyrite is often present in the shales and sandstones that surround the coal seams as well as in the coal itself.

Pyrite is easy to recognize. It has a brassy yellow metallic luster and often forms cubical crystals with sharp edges (you may need to look at it with a magnifying glass to identify these edges if the rock has been tumbled around in a stream

It usually occurs as spots or streaks on the surface of the rock. If the specimen is weathered, i.e. has been exposed to rainwater or stream water, there will sometimes be a halo of rust around the pyrite crystals.. This is because the sulphide is gradually converted to oxide by the weathering processes. Here is a case where 'rusty' rocks are definitely not harmless. However, the residual presence of brassy flakes of pyrite will be an obvious warning in such circumstances.

Above is a picture of a rock with a lot of pyrite mineralization. Note the brassy metallic luster (fool's gold), the sharp edges of the pyrite crystals, and the incipient iron oxide (rust) discoloration round some of the pyrite.

You would obviously reject this rock out of hand--or keep it on a shelf as a mineral specimen. Pyrite crystals in rocks such as limestone or slates may be less spectacular, but they are still obvious.

Clearly you should never put any rock with visible pyrite into your tank.

In conclusion, observe the common sense approaches above and you will never have to worry about placing a rock into your aquarium again!
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