Goldfish tail splitting - help!

[ilfeax]

Hi there,

I'm new to the forum, nice to "meet" everyone! I am posting because our comet fantail goldfish, who we have had for 10 years, has developed a tail splitting issue. We have done a few cycles of medication (Maracyn 2 & API Fin and Tail Cure) plus 20% water changes and we just can't seem to get rid of the infection -  his tail will start healing, and then develop a new split. Besides the infection, he is pretty lively and eats plenty. Does anyone have advice about how we can nail this thing for good? Any help is much appreciated