People love to trumpet water quality as the root of all evil even if fish show no signs of it but gourami love to attack large or bright fins and can easily tear apart smaller fish.
Maintaining clean water does solve alot of ills and is often overlooked as a cause. Since there are no pictures from which to form a factual opinion, changing water is definitely a plus and should be done anytime you have a suspected ill fish. Clean water is a panacea for many things. Salt too can be of help in certain situations. The clean water aids in healing regardless of cause. This does not suggest that the water is "at fault", but is rather a quick and easy way to eliminate one possibility.
If the tail had been chewed off, I would think it to be reflected as a red, raw, jagged injury. Smooth and without redness would point another direction such as fin rot or caudal disease.
