Main types of stones used for aquarium design.
Granite. A hard rock of volcanic origin, speckled with gray, black, or white, and sometimes orange colors, mainly used for decorative finishing of buildings. There is also a special orange variety. The color of granites varies widely: from light gray to black and from pale pink to dark red. It is practically neutral and is an excellent universal material for an aquarium. Granite is very often used to build a rocky shore. This material does not release any substances into the water. The only inconvenience of its use is its high specific gravity, meaning the rocks turn out heavy. They must be handled with care so as not to break the aquarium glass.
Marbles are formed as a result of the recrystalisation of limestones and dolomites at high temperatures and pressures. During formation, marble was impregnated with solutions of various salts, metal oxides, and other impurities, which gave it a wide variety of colors and shades. The presence of iron oxides, for example, gives marble reddish, brownish, and pink tints. Microscopic carbon particles found in marble create gray, black, and greenish tones. Very beautiful rocks can be made from marble, but this material is relatively rare.
Basalt is a rock, a very strong stone used for paving streets (cobblestone). Its color ranges from dark gray to black. Most often, basalt looks like this: a dark gray, dense, earthy, heavy mass in which lighter small rectangles of feldspar and bottle-green olivine specks are visible, sometimes with bubbles filled with quartz, feldspar, or lime spars. There are quite a few advantages to such a substrate: low cost, an almost ideal black color, relative availability, and a fairly wide choice of fractions. Disadvantages include the high "dustiness" of new substrate (rinsing can take a lot of time) and its unpleasant tendency to pack down. As a result, the gas exchange process in the aquarium may be disrupted. Some trauma risk from crushed basalt for bottom-dwelling fish is also noted.
Pebbles are small, water-rounded fragments of various rocks of rounded shape. Collected by hand along the shores of seas, lakes, and rivers. They are as close to natural stone as possible and come in a large variety of types and sizes. Pebbles have a wide color palette. They differ in color, rock, and structure. Simple color structures include dark blue, gray, red, and white. As a rule, these are sandstone, quartz rocks, granite, and limestone.
Limestone. A sedimentary rock consisting mainly of calcium carbonate. Some limestones also contain magnesium salts. It has an uneven, cavernous surface. Limestones range from fine-grained and hard to coarse-grained and crumbly. All limestone rocks have a light color: from white to grayish-yellow. May contain fossils. The main component of limestone is calcite, but in addition to it, this rock also includes alumina, magnesite, silica, often mixed with dolomite. This material dissolves slowly, increasing water hardness by 2-5°. In medium-hard water, the influence of limestone rocks is minimal, while in very soft water, hardness increases noticeably.
A rockery with many crevices, caves, and shelters is built in the aquarium from limestone, shell rock, or individual stones. Dense limestone is usually obtained by aquarists in the form of slab fragments. Grottos are built from such pieces. Porous limestone must not just be washed before being placed in the aquarium, but necessarily soaked for 1-2 days. Unwashed limestone can sharply shift the water pH to the alkaline side up to 10-11. In an old aquarium, this can cause mass mortality of fish and plants, while in a newly set up aquarium, special solutions will have to be used to normalize the pH or all the water must be changed. Limestones become overgrown with algae quite quickly, and they have to be cleaned. And given that limestone is a soft stone, rough handling and cleaning noticeably destroy it.
Porphyry is a natural natural stone of granite rock of volcanic origin. Porphyries, like the vast majority of volcanic rocks, have a characteristic structure called "porphyritic". Rocks in which crystals of gray quartz and transparent feldspar with shiny faces are clearly visible are called liparites. During the "aging" process, feldspar inclusions fade, and such rocks are called quartz porphyries. Porphyry has increased strength. Porphyry is not only red, but also greenish, black, brown, purple, or speckled with sparkling iron glitter. Quartz porphyry is an analogue of granite, having a glassy structure with inclusions of large grains of quartz crystals. Along with quartz porphyry, there is quartz-free porphyry, in which quartz is absent.
Lava is a fiery-liquid, predominantly silicate melt poured out onto the earth’s surface during volcanic eruptions, forming a scoria and glassy crust or blocks upon solidification. According to composition, acidic lava and basalt lava are distinguished. Foamy solidified lava is relatively light. Magnificent grottos are made from lava stone. It has a dark brown or reddish-black color. Lava is practically inert to water and can be placed in the aquarium without prior preparation. Unfortunately, this material is quite expensive. But it must be admitted that it is the most decorative and practical.
Gneiss is a very hard metamorphic rock. In composition, gneisses are identical to granites. Structurally, they can be attributed to the group of schistose rocks. Practically neutral. The composition of gneiss is identical to granite, but it is easily distinguished by its characteristic banding (alternation of light and dark bands). An important diagnostic feature is the schistose structure. It is often striped, with stripes usually gray, gray-green, or white, sometimes pink. Good aquarium stones suitable for any purpose.
Dolomite is a natural calcium and magnesium carbonate. For the most part, they are fine-grained. The color is white, gray, often yellowish. Dolomite is scratched with a steel needle and differs from limestone by lower solubility and stronger luster. The calcite content varies greatly, so there is a transitional series between dolomite and limestone.
Sandstone. A common sedimentary rock, usually clearly layered. It is formed by the cementation of sand grains with clay or flint. Its composition, and consequently its effect on the chemical composition of water, is determined by the composition of the sand. Some sandstones increase water hardness and pH. There are soft and brittle sandstones, they are not suitable for use in the aquarium. Depending on the binder, sandstones have different colors. The color of sandstone can vary from white (light gray), beige to red, brown, and even green and blue. There are many forms and varieties of sandstones, these can be layers with an interesting natural pattern, blocks with uneven edges in the form of rocks, oval shapes, cavernous sandstone.
Cavernous sandstone is blocks of various shapes with numerous through-holes, resembling a sieve. The surface of such a stone comes in two types: rough with holes or smooth with holes. The rough surface has an undeniable advantage and looks more decorative in an aquarium. There is also a difference in color; the most common are white, beige, yellowish, and gray colors. Cavernous sandstone is essentially a natural constructor with the help of which you can build real underwater reefs. You can take a large stone and use a hammer and chisel to knock out the loosest areas, turning the block into a multi-story dwelling with caves, passages, and arches. It can crumble, giving the aquarium an untidy appearance.
If you decide to set up a freshwater aquarium in a marine style, sandstone is best suited for this purpose. This stone in the aquarium can play a very important role for both fish and live plants. For a number of aquarium fish, it will serve as a shelter, especially for cave-spawning fish. Cavernous sandstone is very rich in trace elements such as iron, aluminum, barium, cobalt, chromium, copper, nickel, vanadium, manganese, and zinc. Such a rich composition has a beneficial effect on both plants and aquarium inhabitants. Sandstone is capable of increasing water hardness, and for these reasons, it is not compatible with all fish species. But some fish, on the contrary, like hard water and it is even useful for them, so they are used only in bodies of water for fish whose natural habitat is Lake Malawi and Lake Tanganyika.
Shell rock is a sedimentary rock consisting of compressed mollusk shells and other marine inhabitants. Due to its composition, shell rock has a porous microstructure filled with air. Shell rock wall block is a porous material with a low bulk density, clearly expressed layering, and relatively low mechanical strength, and is easily processed. By color, shell rock is divided into yellow and white. It consists of mollusk shells and fragments held together by a calcareous cement.
Slate. A very hard rock known for its gray color (usually dark gray) and layered structure. A rock having an oriented arrangement of rock-forming minerals and, regardless of composition and origin, characterized by a thin-layered structure and the ability to split into more or less thin, flat, parallel layers or plates. The color of slates ranges from dark gray to black. Some slates contain intrusions of other minerals between layers, manifested as colored veins or layers on the side edges of the slate. Such slates are unsuitable for use in the aquarium. Shales, depending on the main mineral making up their composition, can be: mica, clay, talc, hornblende, chlorite. In construction, clay shales are mainly used. Clay shales, in addition to small clay particles, include mica flakes, feldspar, quartz, and other minerals.
Pure slate is an excellent neutral rock. It can easily be split into relatively thin sheets or plates with flat edges. From various types of slate — anthracite, gray, or light brown — you can stack not only terrace ledges, but also various structures with shelters and caves in front of the back wall of the aquarium. They are especially loved by various species of cichlids from Lake Tanganyika. Slate chips are sharp enough for a careless fish to sustain severe injury. This must be taken into account when building rocky structures. As a rule, they are dark gray, dark green, or bluish-purple.
Tuff is a porous stone rock formed from volcanic ash. Tuffs have a spongy structure. This meaning of the word has also been preserved in relation to carbonate and siliceous deposits of hydrothermal springs, called calcareous and siliceous tuffs. The color palette of tuff is very wide — from black, brown, orange, purple, pink, burgundy, green to light yellow. They have good decorative qualities, low bulk density, and high strength, but can be used (like all calcium-bearing rocks) only in hard water aquariums.
Calcareous tuff. A very rough and brittle limestone. It is popular among aquarists who keep fish originating from bodies of water with alkaline water with underwater stones and requiring shelters. It is porous and light, and large structures can be built from it, resting against the back glass of the aquarium without causing overload. As a spawning substrate, it is completely useless because its surface is too rough.
Quartz. Quartz also occurs in solid masses. Such varieties of quartz are called chalcedony. Quartz is easy to distinguish from calcite. It scratches glass, while calcite does not. Calcite dissolves with a hiss under the influence of any acid, including vinegar, while quartz dissolves only under the influence of hydrofluoric acid, and even then rather reluctantly.
Quartzite is a natural stone, a metamorphic rock consisting mainly of quartz grains that are microscopically indistinguishable from each other and merge into a solid dense mass with a conchoidal or splintery fracture. The rock is formed in such a way that it is easily divided into pieces. Quartzites can be dark cherry, red, purple, and sometimes green. A stone unique in its properties, possessing greater strength and wear resistance than other natural stones. Even granite is inferior to it in these parameters. According to the content of other minerals, quartzites are divided into micaceous, garnet, and hornblende. A variety of quartzite is jasper. This is a rich decorative stone with a patterned design reminiscent of Karelian birch. Jaspers come in various colors: green, cherry-red, olive, buff.
Shells cannot be considered a suitable decoration for a general aquarium, since they consist of calcium carbonate and can affect water hardness and pH. However, certain species of fish (mainly shell-dwelling cichlids from Lake Tanganyika) definitely need spiral shells, which they use as shelters and spawning sites. For this purpose, French edible snail shells (Escargots), which can be purchased in delicacy stores, are usually taken. They are ideal for most fish species. Shells sold as interior decoration are also suitable provided they are not painted or varnished.
Shells from the sea coast must first be sterilized by boiling, and then all their available surfaces must be thoroughly scraped clean. Those shells that have been empty for some time are well polished by water and are preferable to those that may still contain the remains of dead mollusks. Obviously, whatever shells you choose, they must first of all be of a suitable size to accommodate the fish that need them.
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Which stones to choose and where. Tips for choosing stones. 
