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You must not to overfeed. If the fish refuse to eat, no need to add more food. You only need to feed it once a day, with no more than the fish can eat in under a minute. Remove leftover fish food immediately after feeding to avoid spoil the water. Or the leftover food will break down into more ammonia and poisoning your fish.
Your 8.5 gallon tank is too small even for a single goldfish. By adding another bigger one is not a good idea. You will increase the ammonia production. Remember that your tank is not cycled. There is nothing in your tank to reduce the ammonia as of now.
Can't tell what kind of goldfish it is without a good photo. Probably a fancy goldfish most likely.
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