I think it's the beginnings of fin rot, which can be caused by stress or poor water quality. You should use an antibacterial medication and try to address the cause.

 

Hi,

When I see chunks missing, I think fin nipping. I would do some small, daily water changes for a couple of weeks. I've found that keeping the water pristine will help with issues like this. It's hard to figure out the reason for fin nipping because there can be so many causes. Usually they do it out of frustration. He might be seeing his own refection in the glass, or he might not like fasting day, if you have one, or he might not like carrying around that long tail...so many reasons. If you see him swimming in tight circles, or hiding in one area, and leaning up against something, that might be what he is doing.

You can try adding some tannins to the water to help relieve stress. I use Indian almond leaves (catappa leaves), but there are lots of ways to get that nice brown tint of tannic acid. Alder cones, black water extract, or even brewing a cup of organic rooibos tea with treated tank water and adding it slowly to the tank after it cools will help. Tannins will also provide some anti bacterial and anti fungal protection to the water.

If you see his fins getting worse, then I would recommend putting some methylene blue directly on his fins. Methylene blue is very safe and usually helps. I use a small baby medicine dropper when I apply it to my bettas when they have fin or body issues. Just scoop him up in a cup, drain most of the tank water out of the cup, put a few drops on the affected area(s), and then put him back into the tank. A little methylene blue in the tank won't harm anything. Just be careful with methylene blue because it will stain you fingers, your clothes, the counter tops...any thing it comes in contact with because it is a dye.

I usually avoid antibiotics when possible because they can cause internal organ damage. There's so many antibiotic resistant strains of bacteria now because of the over use of antibiotics, that I try to avoid using antibiotics unless it is a last resort. But that's just my opinion.

I would also check out the decor in your tank to make sure that there is nothing with sharp edges that he might rip his fins. Bettas love squeezing into places that they shouldn't be, and sharp edges can really cause some damage to delicate fins.

Hope this helps, and I hope your betta gets better soon!