The maxim Prevention is better than cure is always true which also applies to stopping your goldfish getting ill in the first place, rather than try to cure the illness when it has occurred. There are a number of practices that a fish keeper should take in order to reduce the risk of a domestic goldfish becoming ill.
When fish are added to a aquarium they require special care to get over any stress from transportation and being introduced into a new fish tank. New goldfish are susceptible to goldfish diseases because of this stress. To reduce inital stress, float the bag they arrive in the aquarium for about 20 to 30 minutes so they become adjusted to the water temperature and their new home. Look out for goldfish ich disease also known as goldfish white spot disease after introducing new fish.
Like all wildlife, Goldfish require to be fed using a good quality feed that meets their dietary requirements. A decent goldfish food can be easily digested, limiting the quantity of waste the goldfish release into the water. Poor quality foods might result in a diet that may be lacking in essential nutritional requirements, potentially they might also result in too much waste being released into the tank, silting up the gravel and reducing the water quality.
Every aquarium must have a suitably sized filtration system which acts to filter out solid and liquid wastes. The basic function of filters is to remove waste and pollutants and keep the water clean. All aquariums should have a supply of oxygen to supplement the oxygen dissolved by natural means at the water surface. This is done utilizing an air-pump in conjunction with a tube and an air stone or alternative device to split the pumped air into thousands of lttle air bubbles. A quality air pump will achieve this quietly and efficiently. Gravel at the bottom of the tank should be kept reasonably clean with a gravel siphon because accumulated dirt can reduce water quality. Water requires to be changed at 3 to 4 weeks. Mains water should be treated utilizing a reputable water conditioning product which can be obtained from local pet stores or aquatic centers.