[header] How to ensure your pet fish are healthy [title] Set up and furnish your tank fully, and relax. [step] You don't want to bother your fish by constantly moving around and adding surroundings. For most tanks, a filter, heater, air pump, fake/real plants, substrate, and hiding places are enough. 
Each species has its own preferences, though, so research. Saltwater and brackish tanks are also different from freshwater.