As I went about my daily check of Alex and his tank today, I noticed that the poor little guy had a couple of nasty splits in his tail fin! While this is quite a large area of injury for a small fish, in my experience, this pattern of damage on a fin is characteristic of mechanical damage. The specific type of damage that I see is:
You will note that, while the very lower tip of his tail fin appears darkened, this is a part of his natural coloring that I was already familiar with (from my daily checks!) - it is similar to the blue edge on the bottom of his anal fin.
First of all, I have done all that I can to make Alex's tank suitable for him and his long fins:
Damage like this can be quite common in bettas, especially as their fins get longer. Remember that bettas were bred by humans to have such long fins - they serve no purpose for the fish, which naturally has very short fins.
When Alex came to us, his tail fin was the same length as, if not longer than, his body! Ridiculous! In coming to Betta Late Than Never, Alex upgraded from an unfiltered bowl to a 5 gallon tank. As such, he has a lot more space to swim around in, which he has certainly been doing, and he also has some current. These new factors will likely cause his insanely long tail fin to erode.
I have come to observe injuries like this quite frequently in bettas, and think of it in somewhat the same terms as a fingernail that grows until it is too long, and then breaks. However, an important distinction must be made:
Since I have assessed his tank and determined that I have done all that I can to protect him from water movement, there is nothing more that I can do in that regard. Recall that fish do experience pain, and this injury would be painful. We must do everything we can to help prevent injuries like this.
Though he is not due for a water change for another few days, I am going to perform a partial change (no more than 20%) today, carefully removing any debris from the bottom of the tank. This will ensure that there is no rotting food to dirty his water at all, leaving him with the ideal conditions in which to heal this injury on his own.
Finally, we will continue to monitor Alex daily to make sure that he is healing and that no infection develops at this wound.