Goldfish. HELP! He looks bruised and has a torn fin!?

Did you test with a liquid test kit or the strips? The strips can be very inaccurate

I'm thinking the black patches, torn fins and not eating point more toward water quality

Standard goldfish are considered pond fish, will grow over a foot long, are very fast swimmers, and yes, they can definitely be bullies towards fancy goldfish

If you've isolated the fantail and happen to have an extra filter I would suggest putting some media from the 20 gallon into the isolation tank, along with a little bit of original tank water so it's not a complete shock to its system. Putting it into completely new unestablished water conditions will probably do more harm than good

A little regular aquarium salt or melafix if you have either on hand wouldn't be a bad idea for the fantail

I would also consider re-homing your standard goldfish before it kills your fancy - whether that's from fighting, or the bio overload that's going on in the tank

Good luck. I really hope you get it sorted out and he starts to recover

Goldfish are peaceful no matter what kind they are, they don't really bully each other to death like that.. and if they were, you would have seen it happen at least once. Black patches on your fish and frayed fins usually means that your fish is healing from burns caused either from chlorine or ammonia. Test your ammonia levels again and make sure to add de-chlorinator to you water. If everything is fine in that area just allow the patches to heal, perhaps add some Melafix to the water to help repair the fins and kill harmful bacteria.

How previous is your fish? If he's youthful he might desire to be achieving adulthood and getting his person colorings. If he's previous, he might desire to be changing coloration by way of age. only as human beings get gray hair or canines ears and muzzles turn white.