Fish is really happy and eating one day, dead the next

Well, you said that your one fish had been acting weird for a while. Was he floating upside-down a lot, or swimming in circles? Was he just very sluggish, swimming slowly and acting uninterested in life? Was his body bloated? Did he scrape his sides against the walls of the tank a lot? Was he simply swimming to the top to gasp at the surface? Acting weird covers a lot of territory with goldies. Give us some idea of the symptoms and we can help you a lot better. Goldfish are social creatures but I've never yet seen one grieve itself to death. The chances are really good that your second fish died either from the same thing that affected the first one, but just didn't display the strongest symptoms, or the second fish had some other condition pop up suddenly at the same time that the first fish finally succumbed. Now that both fish are gone, this is a good time to clean the tank thoroughly, including filter media, tubing and any 'toys' that exist in the tank environment. Also, take out any live plants and give them a good rinsing in sudsy water, then rinse in clean water. Clean or replace the bottom media if any is present, such as rocks, pebbles or sand. Start fresh with clean water, clean filter, clean everything and allow the tank to cycle for a week or so. Some may suggest a month, but either works. Don't buy fish again until you have a clear idea of what caused at least the first death and can do something about it should it happen again. The symptoms I've described above are all legitimate goldfish disease symptoms, and many of them can lead either to permanent disfigurement or death. You can go online and google the symptoms exhibited by your fish and see what comes up as a match. Then you can learn how to deal with it if it pops up again. By the way, this may have already been mentioned, but goldies love cold water, so don't use a heater in your tank. Water that is too warm can make the fish act oddly because they are stressed. Stress in a goldfish almost always leads to disease, so that's something to watch out for.