I seem to be having a similar problem.
Looks like my yellow has a bite out of its tail.
I think it is an injury but I guess it could be a disease.
Did a water change yesterday after seeing your problem.
I have another yellow with a similar problem.
They both keep hastling other fish and the other fish attack them but they don't swim away or defend themselves.
They seem happy being bitten.
What is this about???
Any ideas???
Looks like my yellow has a bite out of its tail.
I think it is an injury but I guess it could be a disease.
Did a water change yesterday after seeing your problem.
I have another yellow with a similar problem.
They both keep hastling other fish and the other fish attack them but they don't swim away or defend themselves.
They seem happy being bitten.
What is this about???
Any ideas???