A deformed tail is not going to cause that.
If you can provide a link to the whirling disease you are referring to, I can check it.
Normally if fish develop problems with swimming and spiral through the water, they have a bacterial, protozoan or viral infection in the brain. this is generally caused by poor water quality and a dirty environment. If left untreated the fish usually dies within a few days of showing these symptoms.
The fish in question has apparently had the issue for over 6 months so it is unlikely to be this.
The best treatment for infections in fish is to clean the tank conditions up by doing big daily water changes and gravel cleaning the substrate. If there is no improvement after a couple of water changes then salt can be added. However, if this has been going on for 6 months, then it is highly unlikely to be a disease and sounds more like brain damage that was caused by something along time ago.