Oh hush! It happens to everyone, don't beat yourself up over it hun.. I know how you feel though, all that work to get him and this happens. But it's only a split fin and I believe you will do the right thing so it will heal quickly and smoothly. You didn't know about those plants when you got them, I didn't know either until I had one that was connected with a cave I got and it ripped one of my boy's fins twice before I realized what plant and I cut it off.
Yes, stay on top of the water changes. Do your regular ones, you can add in another 50% during the week if you wish for a week or two to keep it a little cleaner. To help it heal a little faster if you have Stress Coat, you can add in a few drops of that to promote the regrowth. Otherwise, just clean water will work.
Is it a small split or is it large? Usually splits fuse back pretty quickly, it's when it's missing a chunk that it take a little longer to grow back.