Which stones to choose and where to find them.
The best stones are easiest to find in the water or along the shoreline of a body of water, because these stones are constantly washed by water and lack sharp protrusions that could injure your fish. Do not collect stones in areas with mines or quarries where metal or mineral ores are extracted. However, if there are nearby quarries where stones used in construction or for making gravestones and statues are mined, this is a good sign, as such stones must have a uniform structure and contain no inclusions of foreign minerals that could prove toxic. Nevertheless, their potential impact on water hardness and pH cannot be completely ruled out.
It is better to choose darker stones. The best colors are black, dark gray, dark green, and dark brown. These stones provide the best contrast with light substrate. You should collect stones of various sizes and shapes, but it is also good to have some round and flat stones. Large stones for creating the base of the composition should be chosen with a rough surface, pitting, and numerous cracks. An aquascaper or designer, and indeed each of us, will choose a stone with an unusual exotic look. Rounded stones are used to create a river landscape, while flat stones of irregular shape are used to simulate rocks.
We use a wide variety of rock types to decorate the aquarium, but some minerals are completely unsuitable for an aquarium and directly for fish. Most stones can be used in an aquarium, but stones containing lime or marine stones consisting of shell mixtures should be avoided, as they can excessively increase water acidity or hardness. Rocks that release iron, calcium, and magnesium—such as travertine, tufa, granite, marble, and limestone—are unsuitable for the aquarium. Calcium and magnesium increase water hardness and pH. Excess iron in the water inhibits plant growth and promotes algae proliferation.
You should avoid any stones with algae or plants growing on them. If your aquarium is well lit, the color of the stones you collect does not matter—they will soon all turn green from algae. However, if the aquarium is lit within normal limits, you should avoid white stones (or all light-colored stones in general) and look for darker ones. A light stone looks very picturesque against the background of a dark rock pile. Stones should have rounded edges. Therefore, before using any particular rock type, you must determine whether it is suitable for the aquarium or not.
Tips for choosing stones:
• Do not take stones with metallic veins or rust-colored layers (rust streaks visible), nor stones containing colored crystals. However, if you see white or transparent crystals or veins, this is most likely quartz, which is safe.
• Look for stones with uniform coloration, evenly distributed specks, and a uniform structure. Avoid brightly colored stones (unless you are sure they are safe), as such coloring may be due to undesirable minerals. Pieces of ore should never be used, no matter how attractive they look.
• Be careful — before placing stones into the aquarium, remember that some of them may contain a large amount of iron, which must not get into your tank. Stones can introduce heavy metals (iron, lead, copper, and so on) into the aquarium along with beauty and decor. Heavy metal poisoning usually has no external symptoms; fish simply die seemingly without cause. To prevent this from happening in your aquarium, carefully inspect the found stones for spots.
Take and set aside stones that have shiny metallic-colored flecks, “veins,” or rust-like “vessels” into a separate container. Pour a weak acid solution (citric acid or vinegar) onto them. If you observe fizzing or the appearance of tiny bubbles on the stone’s surface when the acid hits it, this is a clear sign of metallic inclusions, meaning the stone will leach heavy metal ions into the water.
• Do not forget that matte, grayish-faded stones are most likely limestone, while stones resembling sand in color and structure are likely sandstone.
• Remember that stones containing fossils are generally limestone.
• Stones containing lime or marine stones consisting of shell mixtures should be avoided, as they can excessively increase water acidity or hardness. Tufa, granite, and gravel are free from these drawbacks.
• Do not use soft or crumbling stones (except for calcareous tufa) in aquariums with alkaline water.
• Pay attention to the weight of the stones. On average, a stone is 2.5 times heavier than water. If you have a bucket or basin with volume indicators among your household containers, you can perform a simple experiment. Fill this container with water, submerge the stone, then take it out and see how many liters of water are left. Typically, multiplying this figure by 2.5 will easily give you the weight of the stone in kilograms. The reverse experiment (taking the stone’s weight and dividing it by 2.5) will show how many liters of aquarium water the stones will displace.
• When choosing stones, ensure they do not have sharp edges (fish can injure themselves on them).
• Among water-polished stones, choose those with a uniform surface lacking holes or cracks. Large, rounded pebbles do not look good. Do not take stones that have been clearly cut or broken.
• It looks unappealing to use stones of different colors and textures in a single aquarium. Excessively colorful stones are distracting and disrupt harmony in the tank. Black river stone gives the aquarium a cold feel. Red stones will stand out too much because there will be multiple contrast levels: color, shape, texture.
• It is very important to use different sizes. For the overall impression of the aquarium, the individual shape of a stone is less important than the effect of a stone group. A single stone, even a very beautiful one, looks like a foreign object in the aquarium. However, a group composition can give individuality to the aquatic environment. Thus, an individually unremarkable stone can look great within a group of stones.
• Avoid stones with a top wider than the base (except for branching types).
Stones, just like gravel, should be tested for pH changes. You can determine whether a rock contains lime in a very simple way: drop hydrochloric or sulfuric acid, or a pH-minus solution onto the stone. If foam forms, the rock is calcareous and unsuitable for a soft-water tank. Rocks made of dolomite, limestone, shale, marble, shell rock, seashells, coral, stalactites, and certain types of tufa are used exclusively in hard-water aquariums because they increase water hardness.
Limestone is a sedimentary rock consisting mainly of calcium carbonate. Some limestones also contain magnesium salts. Limestones range from fine-grained and hard to coarse-grained and crumbly. They raise or buffer hardness and pH.
If you do not feel confident enough in your abilities and knowledge to collect and test natural stones yourself, it is best to turn to pet stores. Some shops sell artificial stones. Some artificial stones are remarkably similar to real ones, although their range of sizes and shapes is quite limited and they are quite expensive. However, just like real ones, they will last you a lifetime.
Main types of stones used for aquarium design.
Choosing and preparing stones for the aquarium. 
