Originally Posted by Pataflafla
Looks like he either got snagged on a filter or decoration or he's nipping his fins.
Go over your decorations with a stocking. If it snags, it'll rip fins.
If he's nipping his tail, make sure you have a set schedule for night and day that you don't stray too far from. Make sure he's got things to hid behind/in and explore as well as time to exercise any excess energy.
A hand held mirror is a good tool to get some exercise and boost self confidence. Make winning easy at first, but slowly work them up to a minute or two of flaring before taking the mirror away.
Plastic bottle caps are also a good source of entertainment. You can use different colors to see which your betta prefers.
Everything in there is soft enough for him, the schedule is a little iffy but he does have things to hide in and behind, he has a silk plant with large broad leaves. I might be getting some live plants for the tank, I'll look into the mirror, and I just put a baffle back on to the filter.