Fin rot

Fin rot is most often observed in aquaria and aquaculture, but can also occur in natural populations.

[1] Fin rot can be the result of a bacterial infection (Pseudomonas fluorescens, which causes a ragged rotting of the fin), or as a fungal infection (which rots the fin more evenly and is more likely to produce a white "edge").

Infection is commonly brought on by bad water conditions, injury, poor diet, stress, or as a secondary infection in a fish which is already stressed by other disease.

If it does reach the fin base, the fish will never be able to regenerate the lost tissue.

Fin rot is common in bettas due to poor water conditions in pet stores.