For example, if your water is hard and alkaline, you're best off selecting fish that thrive under such conditions. Ammonia is toxic in low concentrations and severely stresses fish under ANY concentration. Although significantly less toxic than ammonia or nitrite, nitrate also stresses fish. The water temperature of your tank should match the needs of its inhabitants. Keeping water temperature too cold or too warm for a particular species will stress those fish.
Some fish prefer soft water, others prefer hard water.
Other species are extremely intolerant of salt. Add salt only if all of a tank's inhabitants can tolerate salinity. Mollies, for example are known to like salt, whereas many species of catfish tolerate no salt at all. The amount of physical space required for a particular fish depends on its species. Some fish do just fine in a 10g tank, others need 100g or more.