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Aquarium Rock FAQ

Why rocks, aint rocks!

This article serves to provide a brief introduction to rocks and their properties, and hopefully guide you in your choice of decor for various biotope simulations. All text and photos (except fig. 8 ) are © Andrea Watts. Do not reproduce them without permission of the author.

Rock Types

To make a decision about rock selection for aquarium use, it is beneficial to understand the properties of each rock group. The following is a skeletal lesson in geology that will (hopefully) help you understand why some rocks are less suitable for the use in home aquariums than others. Only the characteristics that affect freshwater environments will be discussed.

Rocks are grouped into 3 major types: igneous, sedimentary and metamorphic. Each of these types consists of subgroups. We will start with igneous rocks.

Igneous Rocks

These rocks are formed from molten material (magma): those which contain high amounts of quartz (SiO2) are termed “acidic”; those without quartz are termed “basic”. Rocks with small amounts of quartz are called “intermediate”. Common types of acidic rocks include granite (fig 8 ), rhyolite, pegmatite and obsidian. Intermediate rocks include syenite, diorite, andesite and trachyte. Those that are termed as basic include gabbro, dolerite and basalt.

Generally speaking, these rocks are suitable for use in the aquarium. However they form sharp edges along broken faces, so care should be taken to smooth off any dangerous projections. Obsidian is glass-like and should be treated with extreme care. Basically, igneous rocks are non-porous, however many contain some minerals that are chemically altered to clay after a period of weathering. With the exception of obsidian, some are then capable of “sponging” up chemicals/toxins from their environment and releasing them slowly into the aquarium. Water worn and polished specimens possess little to no absorption properties, and often appear more realistic in an aquatic environment.

Basic rocks often contain higher amounts of iron-based minerals; however their release is too slow to cause a significant shift in the water chemistry. Igneous rocks do not help buffer water, are generally inert and are non-contributors to providing ideal water chemistry for alkaline dwelling cichlids. They provide decorative properties only.

Sedimentary Rocks

Most rocks chosen by aquarists fall into this category. The main feature that brings this great variety of rocks together is that they are deposited in layers or beds that originally are reasonably flat and well organised. Those that are formed from detrital sediments include sandstone, conglomerate, breccia and shale. Sandstones can vary greatly in their chemical composition.

This is due to the fact that the sand is cemented together by various minerals which have been formed in the pore spaces in the rock. The most common “cements” are calcite, iron or quartz. Sandstones, on the whole, are inert in water, although the cement type will dictate their strength and mineral composition (fig. 1).

The most important factor to consider when utilising these rocks is their porosity and often fragile nature. Sandstones bonded with iron will remain stronger over time than those containing calcite: the latter tending to crumble after a period of submersion.

The high porosity of sandstones leads to the containment and release of toxins/chemicals acquired from an aquatic environment. You need to be sure of their source. Conglomerates and breccias are rocks that are formed by”clusters” of larger sized particles (fig. 2). They are cemented in a similar fashion to sandstones.

Shales and mudstones are generally not suitable for aquarium use. They are clay based and often fall apart after a period of submersion.

Limestones are the most commonly used rock group in home aquariums. There are a number of types of limestone, and their properties correspond with the origin of formation (fig. 3.). Some are produced by or from organic material (shells, coral or algae), others originate from chemical activity (oolite and dolomite) and the rest are formed from fragments of calcareous material (clastic). Although limestones can be deposited in freshwater, the vast majorities are marine deposits. They are commonly formed in a reasonably clear sea, largely free of mud and sand.

These rocks are usually pale coloured, being grey or even white, the colour though will depend on the amount of detrital material present. They may be brownish when iron minerals are present and almost black if they contain high levels of mud and organic carbon. The percentage of calcium carbonate present differs between types (chalks containing up to 90%) whereas dolomites contain over 15% magnesium carbonates.

Limestones are useful for use in hard-water, alkaline biotope simulations such as Malawi and Tanganyikan. They possess good buffering properties, are relatively easy to come by and provide aesthetically pleasing displays with a little imagination. Limestone will often break and form quite sharp fractures. Make sure that you do not leave hazardous projections that may injure your fish.

Evaporatives and ironstones include salt, gypsum, potash ore marl and ironstones.

Only the ironstones are suitable for aquarium use. As suggested, they often contain the following iron-rich minerals: chamosite, limonite, siderite, hematite and magnetite, and are commonly added to planted aquariums to help supplement iron levels. Iron content should be regularly monitored if ironstone is included in your aquarium (fig. 4, fig. 5 and fig. 6).

Metamorphic Rocks

Generally speaking, these rocks are formed from the alteration of igneous and sedimentary rocks through heat and pressure. Examples include hornfels, metaquartzite, marble, slate, phyllite, schist and gneiss. All of these rocks are suitable for use in the aquarium. However, as has already been discussed, their origin will determine the chemical properties that they individually possess.

Of these, marble is the most commonly used metamorphic rock. This rock develops when limestones are intruded by magma, or overrun with lavas. The heat from the molten igneous rock brings about profound changes in the original limestone. Marbles are generally pale coloured rocks of medium to course grain size. The main mineral in marble is calcite; the other metamorphic minerals develop from impurities in the original rock. Due to the thermal alteration of the original limestone, marble is very hard and durable. It is commonly used as a substrate in hard-water Africa aquariums, or as a filter media. It has an excellent buffering capability, and provides a more environmentally friendly alternative to coral products.

General Themes

Water becomes hard by dissolving soluble salts from the rocks or soil over or through which it flows. Some rocks, for example, slate, granite and gneiss, contain little or no soluble material and have a negligible effect. Others, most notably limestone, are quite the opposite. Hence rocks may affect water chemistry: corals and shells are largely calcium carbonate and some gravels often contain fragments of these substances. Hardness free rocks are a pre-requisite of the soft-water aquarium.

A point not often realised is that some rock surfaces are too rough for use as a spawning substrate by many species. Rocks that would be suitable include granite, schist, gneiss, slate (fig. 7.) and sandstone.

Rock is sold by weight and can be expensive, so you may be tempted to collect your own - but don’t do so unless you are able to identify different types of rocks and spot any contaminants in them. It is an offence (by law and on the environment) to collect rocks from the ocean, river courses, bushland and the like. Heavy penalties are incurred for this sort of action.

All rocks must be thoroughly cleaned and scrubbed to remove soil particles and other foreign bodies. Small pieces can be boiled for 10 minutes, but remember to allow them to cool. The amount of rock to be used will depend on the biotope you are trying to replicate, and may range from scattered stones on the floor of a forest stream, to large and complex rock piles representing areas of rapids or a rocky lake shore.

Stones can be used to support raised terraces of substrate material. Never position large rocks on top of the substrate - always bed them in so that substrate slippage or fish excavations cannot undermine them. Always make sure rocky structures are solidly constructed so that they cannot collapse, crushing fishes or crashing through glass; consider sticking them together with silicone sealant for added security and stability.

Never forget that in choosing and arranging rocks you must always keep the fishes’ requirements - water chemistry, shelter, swimming space, spawning sites and so on - in mind, and be prepared to forego any ideas which may please your eye, but cause them physical or psychological discomfort. Your aim should be to provide them with a replica of their natural environment in which they will feel at home. They reward you by looking their best.


Figure 1: Diversity amongst sandstone is vast. Colour, texture and mineral composition is dependent upon their origin.

Breccia conglomerate
Figure 2: Breccias and conglomerates consist of larger rocks and particles cemented together.

Figure 3: Variation amongst limestones. The darker pieces contain organic carbons, lighter pieces are of marine origin and those with pinkish colouration have iron in their composition.

petrified wood
Figure 4: Petrified wood is difficult to source and relatively expensive, but gives a dramatic impact when used effectively

moon rock
Figure 5: Quartzites are silicate based inert rocks. The above is an example that is commonly sold in LFS as ‘moon rock’.

Figure 6: An intricate piece of ironstone that contains a high manganese content. These decorative rocks can be striking if used correctly.

Figure 7: Pieces of slate are often used to create flat spawning sites. They can also be stacked to create a terraced effect.

Figure 8: Pieces of granite used in a Tanganyikan aquarium. Collected from private property near Cooma, NSW.

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