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 week 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 sp. will eat Vallisneria. Tropheus from Tanganyika and most of the mbunas from Malawi will graze on algae that grows on rocks.