From my experience with bullfrogs they can be hard to keep inside.
#1- Bullfrog tadpoles thrive in fish ponds outside, you must have a wild pond very close by and a bullfrog layed eggs in your pond or a frog "moved" into your pond, OR you somehow purchased a small tadpole with your goldfish and it grew unnoticed in the pond, bullfrog tads are commonly sold in with the goldfish.
#2- In captivity, 1 adult bullfrog needs 20 gallons or more, they hop so hard, if they arent given enough room they knock themselves against the tank so hard and so often they can break their bones or have serious head trauma. They like deep water to swim in, but also need land to rest (more water than land)
#3- Bullfrogs will eat anything, but sometimes they do poor in captivity and wont eat and will starve to death, especially tadpoles you raised that turn into tiny bullfrogs. But being yours is from the wild, most likely it will have a hearty appetite.
What to feed looking at its size- large crickets,minnows (not goldfish they carry so much disease),silkworms,large earthworms,cockroaches (not the pest roaches in your house!),waxworms as a treat they are very fatty and bullfrogs will always beg for food and become overweight, even pinkie mice (frozen), I guess it would have to thaw out and you would have to move it in front of the frog so it sees it (but you DONT NEED to feed mice the other stuff is fine given variety)
It will eat your fish if kept in the same tank!
#4- Bullfrogs do hibernate, so if you release him now he will die, it's too cold for him. Thats why he was under the mud at the bottom of your pond, he was hibernating.
You could try keeping him until next year when you see the wild frogs come out of hibernation.
This site is good on basic care, though when they say a 10 gallon is ok thats incorrect.http://www.grizzlyrun.com/Pets/Amphi...og/Default.htm