Cleaning Your Tank
If there is any area that most goldfish owners are particularly lazy at, then it's got to be cleaning their tanks, bowls or aquariums. Now unless you have a state of the art filtration system (seen usually in commercial aquariums), then you will be required to clean your tank once a week! I know, I know, it sounds like a lot of work, but if you decide to leave it for any longer, you're giving certain harmful bacteria a chance to multiply and mutate, which will eventually affect your goldfish.
When cleaning the tank, a lot of people make one more mistake, they don't wash their hands! Make sure you always wash your hands with an antibacterial soap before you even think about handling your goldfish! Human hands are a breeding ground for incredibly harmful bacteria of which could be lethal to your goldfish.
Cleaning Your Filter
I want you to go and take a look at your filter and check to see how clean it is? Is there any unsightly fungus inside or outside of the filter? Because if there is, it's likely that it's spreading disease through the tank. Make sure you always clean and rinse out your filter every time you change the water. Filters can be a breeding ground for bacteria that cause mostly Ick, which is one of the most common diseases associated with goldfish.
Get Yourself a Water Siphon
Water siphons are used to clean the gravel in a tank. They work by sucking out all the bad particles that build up over time and should be used thoroughly twice a week to keep conditions in your tank optimal for living conditions. If you do happen to have a siphon it means you won't have to completely change the water in your tank week after week. It is recommended that you still should try and change at least 25% however, to keep things nice and clean.
In the end, an unclean tank is the main reason why goldfish tend to contract certain diseases, so make sure you keep it looking great, for your fish's sake!