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A quarantine tank is important as it keeps a fish away from the rest of your aquarium. It is highly reccomended you use a quarantine tank before acclimating your fish to your main aquarium. If a fish you just bought is sick, you do not want the infection to spread to the rest of your tank. After buying a new fish, purchase another tank to use as a quarantine tank.  You do not need to get a fancy tank. A simple 10 to 20 gallon tank should be big enough for a quarantine tank. You can purchase an aquarium at a pet store. You can also order one online. As with a regular aquarium, your quarantine tank should have some kind of filtration system. This will keep your fish safe and healthy during the quarantine period.  If possible, go for a tank with a built in filtration system. If your tank does not have a built in system, purchase a filtration system at a local pet shop. Install it in the tank when you get home, following manufacturer's instructions. This will keep the water at a safe temperature for your fish. You should also get a thermometer to record the temperature. You want to make sure the temperature is safe before transferring your fish to the quarantine tank.  An aquarium may have a built in heating system. If yours does not, you can buy one at a pet store. The proper temperature depends on the type of fish you have. Ask at the pet store what a safe temperature for your fish would be. The quarantine tank should be similar to your regular tank. Once your fish is ready to be transferred to the regular tank, you want the transition to be as smooth as possible.  Take water from your main tank, using a bucket or cup, and fill up your quarantine tank. Once the quarantine tank is full, you can turn on the heater and filtration system. Watch your fish closely during this time. Before introducing a fish to a tank of other aquatic life, you want to make sure it does not have any diseases. Diseases can spread fast in a fish tank.  Common infections include things like fin rot, vibriosis, and mouth rot. Usually, you would treat infections with antibiotics. Antibiotics may be administered to a fish's tank, or you may give your fish food with antibiotics. Symptoms of infections changes in color, frayed or rotting fins, a lack of appetite, gray marks on the scale and fins, and open sores on a fish. If your fish has an infection, make sure it is treated and symptoms are gone before transferring your fish to the main aquarium. If two to three weeks pass without incident, you can transfer your fish to the regular tank. You would repeat the same floating process you used to acclimate your fish to the quarantine tank.  You will have to net your fish and place it in a bag filled with water from the quarantine tank. Make sure to seal the bag with a metal clip or rubber band. Float the bag in the regular tank for 15 minutes, cut the bag open, and roll down the edges an inch. Transfer half a cup of water to the bag every 4 minutes until it's full. Discard half the water in the bag, and float it in the tank again. Once again, transfer water every 4 minutes until the bag is full. Net the fish and transfer it into the main tank.

Summary:
Get your tank. Install a filtration system. Add a heater. Fill the aquarium with water from your main tank. Monitor your fish for 2 to 3 weeks in the quarantine tank. Repeat the floating process to transfer the fish to the regular tank.