Sometimes you can get a bit of infection tht doesnt go further but will infact cause fin to deform slightly, its like a small white ball at bottom of split.
This happened to an ochiba of ours over the last winter to its right pectoral. Surgical intervention then has to take place to correct this .
Simply put your koi under anesthetic and with a shape scaple nip it out and paint with Malachite and Propolis.
Other times they just wont meet again you can surgically intervene by stitching the two halfs together again you put your fish under gently stitching the to halfs together. .
The most dagerous splits however are those that meet with the quick i'e dirctly were the caudial fin joins the body of the fish , these should be watched like a hawk.
They have a rather nasty habit of becoming infected big style, (normally down to bad water conditions/bacterial infection because of this).
Now any that do can rot the caudial fin quickly away, damaging the area that forms the indivual rays of the fin grow from (this can be any fin be it caudial dorsal pectorial pelvic or anal).
"You are in deep trouble if this happens", especially on the tail.
They can be turned around with antibiotic intervention but with massive "unsightly" permanent loss of finnage.
However Dave no caudial fin no fish, as it leaves them unable proppel itself forward through the water without a caudial fin.
Any other fin you can get away with this but not the caudial fin its an instant death sentance for the poor fish and the kindest thing to do.
Sadly the number of these that people have asked me to go look at or have been brought to our door be they koi or goldfish over the last 26 years of koi keeping, I have lost count of (all were avoidable) non knew what they were doing.
So heres a lesson to be learned with the answers to them I hope this helps you in future damaged fins something koi are quite good at doing .
Oh I forgot one the divit, ding or lump as people have described them as before they are little individual white lumps normally, you'll see mabe one or two its where the koi has hit something not splitting the fin, these though nsightly quickly grow out . Look at them as when you hit a car fender and dent it, the dent goes inwards and the mechanic has to knock them back through, yes ? Well these exactly the same but with no mechanic on hand to fix them the koi simply grows them out leaving no sign of any damage lol.
Man I love that least description its very apt....
Dave
