There are other chemicals in tanks besides those you test for with a master test kit. One tank might have different phosphate levels or soft vs hard water. A dragon goby is a hard water lover. But this case sounds like moving an oscar. Oscars love to throw tantrums. That's definately an overdramatic fish you've got there. He should be in a tank 75 gallons or larger, so you definately need to get him out of that 10. Does he have a favorite decoration you can move to the new tank to see if he hides in? Maybe move a few of the new tankmates in with him in the 10 gallon so he can get used to them? You should get rid of the feeder fish altogether assuming its a goldfish, it belongs in a pond with a few hundred gallons.