The number one discussion topic for goldfish people seems to be fish health and diseases. I consider this unfortunate, as it is not my favorite subject. But I've had my share of problems just like everyone else, so I recognize the importance of disseminating information and experiences. I think it is important to try to familiarize yourself with the basics of aquarium ecology; many goldfish diseases can be avoided by just meeting the minimum acceptable levels of care and hygiene. Despite what you read, goldfish are pretty darned hardy if you meet their needs.
Here is a test. Read the following paragraph:
"We bought 4 feeder goldfish a week ago along with a two and half gallon plastic goldfish tank. We feed the fish all the flake food they can eat in 10 minutes twice a day, and change two cups of water each day. Based on the pet shop's advice, we are adding Amquel, Stress Guard, and GoldOomed to the water."
If this scenario sounds in anyway plausible to you, read the basic care information on this site, and then look at my book reviews and find a goldfish and/or fish health book and read that. Incidentally, I definitely don't mean this to be insulting; there was a time not long ago when I would have seen nothing wrong with the test paragraph above. I also do not intend it to disparage the products mentioned; they have their appropriate uses. The point is that there is some basic knowledge required in order to successfully keep goldfish. Take it as a challenge to acquire this knowledge!
Here is some additional information:
If you are reading this because you have problems, I hope this is helpful. Again, make sure you are familiar with the basic care needs of goldfish. Here are some additional sources of information on goldfish diseases and treatments:
This is one of the more interesting areas of goldfish keeping for me. I've learned a lot about animal (including human) nutritional requirements in the process of studying goldfish. In this section I want to offer information on feeding goldfish, their nutritional requirements, and food recipes and ingredients suitable for the goldfish diet. This information has been gathered from a number of sources: most are non-technical. If someone has information from an experimentally validated reference, please pass it along!