Not really a "rank" so to say as much as just an indicator of how many posts you have started or participated within. The more fish you have, the more active you are on the boards. The difference between the blue fish and the yellow fish is I believe another indicator of overall board activity.
I could be TOTALLY off on this though, but I'm in the ballpark at least!!
you can also get a tang lit up by refering others here by emailing them your referral link. and you must have to post alot more than a hundred because i did not get a tang lit up for that not even at 200. Only for the one referral i had. Maybe i'm owed a tang