Quarantine Freshwater Fish
1) You will need a quarantine tank to put the unhealthy fish in. This does not need to be an expensive one, but large enough for the fish to be able swim around during the quarantine period.
2) Make sure the tank is fully cleaned prior to filling up with water. Only use hot water to clean the tank as soaps and detergents will pollute the water.
3) I would always advise filling up the quarantine tank with water from you main tank. An unhealthy fish is already stressed and new water may just kill it off. Use it as an opportunity to do a water change for your main tank. I would advise plenty of hiding places in the quarantine tank so that the unhealthy fish can hide away when stressed.
4) It is advisable to test the water chemical levels with a testing kit. These can be purchase from any good aquatic store. If the levels are not right then they will need adjusting if you can.
5) The quarantine tank will need to have everything your main tank has just on a smaller scale ie a heater, filtration and a light source. I would run the tank for a good couple of weeks and maintain it at the same temperature as your main tank. And your quarantine tank is now ready. Any serious fish keeper will have a quarantine tank ready so the if fish become ill they can be separated from the main tank. Plus when a new fish is purchased it can be housed in the quarantine tank for a couple of weeks prior to putting it in the main tank. This way if the new fish is unwell or carrying any harmful bacteria then it will not wipe out your new tank. I have personally had a tank wipe out a couple of times both with marine and freshwater fish. It is very upsetting and can be easily avoided. I have learned my lesson.