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.
I have to admit it was poorly thought out. I'm use to feeder fish being quite sick and assumed most wouldn't make the first night which is why we picked up so many to begin with. Most never lasted the first few hours and died in the bag while we floated them.
I have a question - the mosquito fish gave birth and most of the young are cruising around the very bottom of the tank just above the gravel. They are TINY - how do I clean my gravel? Or how long can I go without cleaning it while they grow?
I found some scaping material I gathered in Death Valley and hopefully I'll be making it to the local aquarium store today to check out plant availability. Pretty stoked to get this 20 gallon going.