Most health problems originate with poor water quality. If the fish are swimming in water that has the wrong pH or high ammonia, then they can become stressed and run down, thus making them vulnerable to attack by the parasites, fungus and bacteria that are always present in water. The key to healthy fish is therefore good water quality. Be careful not to overfeed, do regular partial water changes, and make sure your filter is working effectively. If the fish do become sick, then daily partial changes of a third of the water can help, as well as various medications.
I agree with mindemae, hope it is not too late