12 feeder fish in a 10 gallon? That's alot of fish in that size tank, they could have all died from ammonia spike. You should add fish gradually to an tank that has completed the nitrogen cycle. Goldfish can get pretty big if they live. You could probably keep 1 or 2 in that size tank for a while, but they would be better off in a bigger tank. Goldfish are very messy as far as eating and producing waste, so siphoning the gravel and water changes are important.