Ughhhh poor Goku =( he's missing part of his pectoral fin on his right side. I don't have a doctor tank or anything setup. :( I heard betta max is good for healing... and like a tablespoon of salt in a 10 gallon?
Its a little swollen on that side, like he got stuck to the filter, its a little puffy around where his fin meets his body.
He's using whats left of his fin today, yesterday he had it clamped to his side when he swam, now he is trying to use it as much as he can I think :( he has a hard time swimming. I had to move some plants cause he would get stuck.
I would find something to put him in to make a QT tank and start treatment right away. Anything will work that is half to 1g in size, even one of those little betta cup will work.
Start making 100% daily water changes along with aquarium salt 1tsp/gal for 10 days and lower the water level so he doesn't have to struggle to swim, feed him a high protein diet.
If you have IAL or oak leaves add them too
I like to pre-mix my salt water and steep the oak leaves in a clean 1g jug to make dosage and water changes easier.