Fish health is primarily determined by the quality of their water, diet, and their stress levels. Fish are uncomfortable with change, and any major swings in water temperature or concentrations of dissolved chemicals likr pH can be a major hazard to your fish's health. Most fish prefer a water pH between 6.8 and 7.2, and typically do best in water temperatures between 68 and 86 °F. Fish that come from colder climates thrive in colder water, anywhere from 50 to 64 °F. Tropical fish naturally like warmer water that's somewhere between 71 and 89 °F.
New fish will need to be acclimated to your water before being added to your system in order to minimize stress. This can be done by floating the sealed transportation bags that the fish came in within the tank itself, allowing the temperature to gradually change. After 15 minutes, add a small amount of tank water to the transport bags, and wait another 15 or so minutes. This should be enough time for the new fish to get used to the unfamiliar waters, and they can be released into your tank from there.
