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See how your fish swims, breathes, eats and interacts with other fish. Get an idea for what is normal so you know when something abnormal is going on. A healthy fish has a decent appetite and is an active swimmer in your tank. You will need to research specific requirements for the tank size, temperature, maintenance, equipment and food needed to keep the type of fish you have healthy. Salt water and fresh water fish have different requirements. Saltwater fish need more maintenance and are not as hardy as most freshwater fish. Their water composition must be checked regularly. You will need special equipment, such as a hydrometer to regularly measure the specific gravity of the water as well as quality salt mixes. A low-stress environment is the best way to keep your fish healthy. When fish are stressed, their immune system is weakened so they are more susceptible to disease. Preventing stress through regular maintenance and care will go a long way to protect your fish from getting sick.  Regularly maintain the tank through partial water changes. Change about a quarter of your tank's water every other week. Feed your fish a nutritious, varied diet. Most fish can do fine and even thrive on processed flake, stick or pellet food. Adding some variety in their diet will help add some nutrition and fiber, such as frozen or freeze-dried bloodworms, live or frozen brine shrimp and some vegetables. Do not overfeed your fish. Feed your fish only what they will eat in 3 minutes. Not only can excess food foul the water, but it can also make your fish sick. Check to make sure your filtration system is working effectively. Filters remove harmful toxins such as ammonia and nitrites from the water. Give your fish enough tank room to live comfortably. Don’t overcrowd them. A good rule-of-thumb to follow is no more than 1 inch (2.5 cm) of fish per gallon. Put only compatible fish species together. You don’t want your fish to eat, hurt or aggressively compete with each other. Peaceful fish will be stressed if kept with aggressive fish as well as fish species that communicate differently through body language. Your tank’s temperature needs to match the needs of your fish. Keeping the tank too cold or warm will stress your fish. For example, goldfish prefer temps under 70 °F (21.1 °C), while most tropical fish need the water to be around 73–79 °F (23–26 °C). Fish that have been living in an overpopulated, dirty tank will be under stress and could carry disease that might infect all your fish. Spend a little more for quality fish rather than fish that could die less than a month later.  The store's fish tanks should be clean with active, brightly colored, unstressed fish. The store should offer return policies on fish and provide full credit on fish that die the first few days after purchase. Sales staff should be knowledgeable about fish as well as tank set up, sizes, inhabitants, diseases, ect. Generally, stores that specialize in aquariums and fish are best. Adding fish directly to your tank can stress fish and even kill them. The water in your tank and the store tank likely differ in temperature and water composition. The fish needs to gradually get used to its new environment.  Keep store water out of your tank because it can carry diseases or parasites. If you can, quarantine new fish for 2 weeks before adding them to your tank. In this case, get the fish used to the quarantine bowl or tank’s water first—not the main tank. Watch for signs of disease and adjust the water or medicate, if needed. Set the fish bag in your tank. After 30 minutes add ¼ cup of tank water into the bag. Do this every 15 minutes for an hour. If the bag gets too full just remove extra water. Afterward, net the fish and place it in its new tank. For the first few weeks, watch new fish carefully for signs of stress and disease.

Summary:
Observe your fish. Learn about your fish species. Avoid stressing your fish. Maintain water temperature. Get your fish from a reputable store. Acclimate fish before adding them to your tank.