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.
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
Rocks are grouped into 3 major types: igneous,
sedimentary and metamorphic. Each of these types consists of
subgroups. We will start with 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
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
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).
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.
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
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
Figure 4: Petrified wood is difficult to source and
relatively expensive, but gives a dramatic impact when used
Figure 5: Quartzites are silicate based inert rocks. The
above is an example that is commonly sold in LFS as ‘moon
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
Figure 8: Pieces of granite used in a Tanganyikan aquarium.
Collected from private property near Cooma, NSW.
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