As I've read it, a lot of that fighting is for dominance. The largest loach tends to stay the largest loach, and so on down. Loach scuffles sometimes end in the occasional nipped or torn fin, but it's rare for even that much damage to occur. Check your loachs over after a battle for little greyish scuff marks. That's usually about as rough as it gets, and they heal up quite nicely. All part of the rough and tumble life of the loach. I've got two that do it about once a week, and it usually ends with one flicking the mouth hooks out and running away. Seems to say "ok, I'm really upset, if we keep this up, I'm gonna cut you!" and the other large loach seems to accept this as a win.