To provide your fish with the most comfortable setting in the aquarium you should research what their natural environment water conditions are and its habitat. Find out if the fish come from fast moving water, slow moving water, water thick with vegetation, rocky areas, water with lots of wood, or sandy bottoms. Is the water temperature relatively cool (in the low 70's), or very warm (in the mid to upper 80's), or somewhere in between. Most tropical fish will do well in the mid to upper 70's, but there are many fish that may be stressed out at this range and be less resistant to disease.
Water hardness should be considered when selecting fish. Many fish that come from areas of soft water do poorly or will not spawn in hard water, and many fish that come from hard water do poorly in soft water. If you live in an area that has hard tap water, you should select fish that do well in that type of water unless you pre-filter your tap water through an RO system. If you have soft water and you want to keep hard water fish, you will have little problem as long as you add mineral salts to the water.
While the fish may not die in within a couple days when exposed to the wrong pH range their immune system will get weak and they will eventually succumb to a disease. Choose the fish that are compatible with the water conditions and decor they are adapted to.
Fish from the African rift lakes of Malawi and Tanganyika are common in the aquarium hobby. These lakes have unique water chemistry, they are both very hard and alkaline. The mineral content of these lakes makes them unique in the freshwater aquarium hobby. Animals we normally think of as being sea creatures, jellyfish, sponges, crabs, shrimp, clams, and puffer fish can be found in these lakes.
Most of the fish from Lake Malawi, and Lake Tanganyika that we commonly see for sale in the aquarium trade are cichlids that come from rocky reef areas of the lakes. It is very easy to recreate this type of habitat in the aquarium with large river rocks. Vallisneria spiralis or Vallisneria gigantea are two plants that will do well in high pH and hard water. Some cichilds like Tropheus spp. will eat Vallisneria. Tropheus from Tanganyika and most of the mbunas from Malawi will graze on algae that grows on rocks.
Suitable substrate for Malawi and Tanganyika aquariums include, aragonite, dolomite, and calcite. You should not use any substrate that will add ammonia or nitrate to the system as they can bring the pH down.
Malawi fish prefer a pH between 7.8 and 8.6. Tanganyika fish prefer a pH between 8.6 and 9.4. For Malawi fish aquariums I normally aim for a pH of 8.3 to 8.4, and for Tanganyika fish I keep the pH between 8.6 and 8.8. The preferred pH buffer for raising and maintaining the pH this high is sodium carbonate. Sodium bicarbonate will not raise and keep the pH in these upper ranges.
For Lake Tanganyika fish you should have a alkalinity of around 330 ppm (18.48 dKH) and general hardness of 310 ppm (17.36 dGH). For Lake Malawi fish you should have a alkalinity of around 80 ppm (4.5 dKH) and general hardness of 86 ppm (4.8 dGH).
Guppies, mollies, swordtails, variatus, and platies are common livebearers that have been in the hobby very long time and have been developed into many different color strains. These livebearers all come from geographic areas that have limestone as a common rock. Limestone slowly erodes as rainwater flows over it, adding to the hardness and alkaline pH. Many species of mollies are often found near the sea, and in some cases they will venture into the sea. Typically livebearers are found in flowing bodies of water, but some population can be isolated in ponds or lakes.
In the aquarium livebearers do very well in planted aquariums. Plants help provide cover for newly born fry. In an aquarium that lacks predators, livebearers population can become very dense. Many species of livebearers have a short lifespan, on average living only one or two years. To keep a self sustaining population going, filter intakes should have a sponge or nylon netting placed over the intake to prevent fry from being suck into an impeller.
Guppies, platies, and variatus can be kept in aquariums as small as 5 gallons (19 l). Swordtails should have a 20 gallon or larger aquarium. Some species of mollies should have a 50 gallon or larger aquarium. Sailfin mollies (Poecilia latipinna), Poecilia velifera and Poecilia petenensis (a.k.a. Yucatan molly) can grow to 4 (10 cm) and sometimes can become combative.
While it is often recommended to add salt to the aquarium for mollies, it is not necessary as long as the pH is kept between 7.8 to 8.3 with a alkalinity between 5 and 9 dKH and a general hardness at a minimum of 17 dGH, but closer to 34 dGH would be better. If you are keeping live plants with mollies, choose plants that like hard alkaline water.
All these livebearers can benefit from aragonite, dolomite, calcite, limestone, coral rubble, and snail shells being placed somewhere in the aquarium ecosystem. Calcium based material can alternatively be placed in a filtration system away from the main display. These materials will slowly break down in freshwater, adding to alkalinity and hardness.
Labyrinth fishes from Asia are colorful and behaviorally fascinating fish to keep in the aquarium. Most prefer a low flow system, with a calm surface for building a bubble nest for spawning. Many of these fish will show their best color in a planted aquarium. There are many man developed color strains of the many different species of gouramis and bettas.
Floating plants like water sprite (Ceratopteris pteridoides), Riccia fluitans, and duckweed (Lemna minor) are often the choice for gouramis and bettas, as they will use the plants for anchoring their bubble nest. Depending on the size for the fish, these bubble nest can range in size from 2 to 6 inches (5 to 15 cm) or more. Floating plants will need to tinned out on a regular basis, as they can multiply quickly and prevent the light from reaching the bottom. Duckweed can become a pest plant in the aquarium, because it reproduces quickly, and sometimes is difficult to eliminate.
Bettas and dwarf gouramis can be kept and bred in 10 gallon aquarium. Aquariums should be heavily planted to allow fish to hide in case of aggression. Larger gouramis may require a 50 gallon or larger aquarium.
For most of these fish a pH between 6.5 and 7.5 is adequate, with soft to moderately hard water is acceptable.
Most Central American cichlids we see in the hobby come from rivers with moderately hard water. Calcium carbonate rock is common in the area where many of these cichlids are found. The majority either spawn on rocks or in caves. Aquatic vegetation is often very limited in their natural habitat.
In the aquarium Central American cichlids are often kept with little or no aquatic plants. They can be kept in aquariums with floating water sprite (Ceratopteris thalictroides) and other floating plants to help reduce nitrate accumulation. Since many Central American cichlids dig in the substrate it may be difficult to keep rooted plants. Potting plants often will work with these cichlids.
Central American cichlids tanks are prime candidates for incorporating a refugium or hydroponics filtration in the ecosystem. Without out a refugium or hydroponics filtration, nitrate concentration can increase quickly, requiring frequent (weekly) 75% or greater water changes to avoid hole in the head symptom.
Amazon fish we see in the hobby are primarily soft acidic water fish. Hobbyist that keep fish from the Amazon often have wood as decoration. A tea colored water is common in Amazon biotope aquariums. Fish that are often kept in Amazonia biotope are discus (Symphysodon spp.), angelfish (Pterophyllum scalare), Apistogramma spp. (dwarf cichlids), cardinal tetras (Paracheirodon axelrodi), rummy-nose tetras (Hemigrammus rhodostomus), neon tetras (Paracheirodon innesi), plecostomus, and Corydoras catfish.
While many of the fish from the Amazon region will adapt to hard alkaline water, for breeding, soft, low pH water may be required for some species. Many of the fish that come from the Amazon will have much higher hatch rates if the water is soft and acidic. Most Amazon fish prefer a pH between 4.0 to 7.2.
Amazon sword plants (Echinodorus bleheri) make an excellent plant for an Amazon biotope aquarium as they do well in soft acidic water. Amazon sword leaves are often used as a spawning site by discus and angelfish.
Wood is a common decoration in Amazon biotope aquariums. If you are keeping discus or angelfish, providing a few pieces of vertical standing wood will be appreciated, and often used as a spawning site. Coconut half shell with a hole drilled in the side can provide a natural look and be a preferred spawning site for many Apistogramma sp. dwarf cichlids.
If you maintain a Amazon biotope you must be aware of the affect low pH has on nitrifying bacteria. Ammonium reading is common in low pH systems, due to reduced bacterial populations. When a water change is done on a system with a low pH, ammonium can be converted to ammonia if the pH of the new water is much higher than the aquarium water.