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To keep your fish healthy in his environment, you need to keep the tank water clean and toxin free. Fish can release more waste than plants or bacteria can process, and this waste can lead to toxic or harmful chemical build up in the tank if it is not filtered or removed.  If your fish lives in a bowl, you will need to treat the tap water you use in the bowl so it is safe for your fish to live in. Treat tap water with water conditioner and a pinch of aquarium salt  before you put it in the fishbowl. The salt will help to kill bacteria in the water and keep the water clean for your fish. Do not use iodized salt, as this can harm your fish. If your fish lives in an aquarium, you will need to install a filtration system in the aquarium to maintain clean water for your fish. Before you get any fish for your aquarium, you should dechlorinate the tank water and install a filtration system. You will need to allow the tank time to cycle through the water several times, and only introduce a few fish at a time in the environment so the filtration system is not overloaded with waste to process. This will help you avoid “new tank syndrome”, which can kill your fish. If your fish is kept in water that is too warm or too cold, this could lead to high levels of stress in your fish and repress his immune system. This could then leave your fish susceptible to infection and disease. The required water temperature will depend on the fish type. If you have a tropical fish, your water temperature should be around 75 °F (24 °C). Tropical fish are able to tolerate water temperature fluctuations, to an extent. A goldfish, on the other hand, can tolerate water temperatures between 68 and 72 degrees Fahrenheit. The key is to avoid rapid dramatic changes in water temperature and to maintain a water temperature that is comfortable for your fish.  Different species of tropical fish can require different temperatures so always check to make sure you are providing the correct environment. When you buy your fish, the retailer should recommend a reliable aquarium heater to keep the water temperature constant. You can also get a thermometer to confirm the temperature of the tank or bowl water. You should wait a few days after setting up your aquarium to introduce any fish into the tank to allow the water temperature to stabilize. Check with the retailer to make sure the tank or bowl you buy for your fish is big enough as a living environment that is too small for your fish can be dangerous for your fish. If your fish is finding the water temperature too hot, he may display certain symptoms, like darting around frantically or acting hyperactive when it is not feeding time. If your fish appears to move very slowly, seems to shiver, or loses interest in food, the water may be too cold for him. You may need to adjust the water temperature so it is closer to the ideal temperature for your fish type. Adding decorations to your fish’s bowl or tank can help to reduce his stress levels and give him a happy environment to swim around in. Add a plant, live or plastic, to your fish’s bowl. This will give your fish a hiding place, which he will appreciate. If you add a live plant, watch for rotting leaves. You will need to remove or cut these leaves so they do not pollute the water. You can also add rocks and broken clay pots to give your fish more hiding places and make him feel more secure. This will help to remove any waste buildup and any decaying organic matter from excess food and plant or fish waste. Doing a small partial water change weekly will also clean any toxins from the water and keep the water clean.  Do not remove any of the aquarium plants or decorations from the tank or bowl if you don't need to. Removing or cleaning these items can kill the beneficial bacteria that has been filtering through your tank and reduce the quality of your filtration system. As well, you do not need to remove your fish from the tank or bowl when you do a partial water change. This can stress out your fish and could expose him to harmful bacteria.  To do a partial water change, remove 10-15% of the water and replace it with fresh, dechlorinated tap water. You can use a siphon to suck up any gunk in the gravel and decorations. Clean 25-33% of the gravel and decorations with the siphon. You should also use an algae scraper to remove algae on the surface of the tank or the tank ornaments before you remove any of the water.  If your fish tank is under ten gallons, you will need to perform a 50-100% water change at least twice a week, or every other day. If your fishbowl does not have a filter, you should do a 100% water change at least once a day to remove any waste or toxins from the water. Getting a top cover for the bowl or a filter can reduce the water changes you will need to do on a daily basis and protect your fish from getting an infection or a disease.  Check the water once a day to ensure it is not cloudy, foamy, or has an unusual odor. These could all be signs of a bacterial infestation and may require a full water change.

Summary:
Filter or treat the tank water. Maintain the water correct temperature for your fish. Make the bowl or aquarium fish-friendly. Do a 10-15% water change once a week.