If you play music with your fins, one rule is that you must be a fish. You can’t play music using fish fins that don’t belong to you. That’s the rule. And it’s also rude.
There’s also a rule in place to limit the amount of fish fin musicians. No more than ten (てん) are allowed to gather and play their music together. That’s the rule. No more than ten.
Currently there are no similar looking kanji which exist in the WaniKani learning system