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A filter is a vital piece of equipment to help keep your goldfish alive. The filter you’ll need depends on the size of your tank, as filters operate according to a flow rate which will correspond with the amount of water your tank holds.  Ensure you get a filter with the correct flow rate, and choose between an external or internal filter.  If you can’t match your tank’s capacity to a the exact flow rated filter, go overboard.  For instance, if you have a 20 gallon (75.7 L) tank, shoot for a filter rated for 40 gallons (151.4 L). External filters sit on an outside wall of the tank or within the tank’s lid. Internal filters are submerged within the tank.  Either type of filter can work for a goldfish tank. Lean towards opting for an external filter, as they have a greater capacity to store filtration materials, can clean water more thoroughly, and will require less maintenance. Your goldfish requires properly conditioned water to neutralize commonly occurring chlorine and chloramine.  Feel free to use tap water, for instance, but add conditioner from a pet supplies store to make it safe for your goldfish to live in.  Make sure the water has the right pH level too. For goldfish, ideal pH levels are a slightly alkaline 7.2-7.6 pH. Use a pH test kit to test your goldfish’s water regularly, and adjust the pH if necessary with additives available at the pet store. Do not place the tank near a window or any heating or cooling source.  Do not allow sunlight to hit the tank directly.  Also be sure the tank is set on something flat and extremely sturdy.  Though you will likely need to set your fish tank near an outlet, don’t allow electrical cords dangle over your fish tank. An electrical shock could absolutely fry your goldfish, and be dangerous for you as well. Similarly, be sure that no cord is pulling against the side of the tank or the base that the tank is resting upon. Place the tank in a location it will not be jostled. You probably don’t need a heater to maintain a temperature in this range, as it almost identical to the range commonly maintained in human dwellings. If you live in a particularly hot or cold climate, ensure that seasonal or daily changes in temperature do not put your goldfish at risk.
Use a filtration system. Add safe, clean, and balanced water. Place the goldfish’s tank somewhere safe and sensible. Make sure the water temperature stays between 60 °F (16 °C) and 72 °F (22 °C).