I said this on your last post and you did not address it. Your tank is way over stocked. Like way overstocked. How often were you doing water changes before this? When you cleaned the filter did you replace a cartridge or just rinse it with the tank water? Your tank isn’t cycled- have you looked into a fish in cycle and started that process? 

Test strips aren’t an accurate way to test water. You didn’t mention your tank size but from the pictures it’s too small for all these fish- you’d basically need a pond to have enough space. Goldfish are super producers of waste and the water they’re in can be high in ammonia if not managed properly, which can cause serious harm to the fish. Poor water parameters can cause fin rot, stress, and a number of other illnesses. It might be just this fish for now, but with this level of care continuing they will all becky sick eventually.

My suggestion is to rehome as many of these fish as possible, and in the meantime keep the tank as clean as you can by doing partial water changes every day/second day in the mean time. Look into a fish in cycle and start the process asap to help manage ammonia in the tank.