every 10 skill, the skill will cap. you need to pass a test from the guild to be able to gain more skill. The guild will ask you for a fish, you trade it in and you pass, very simple. You can do the test when your skill ends with 8. 8, 18, 28, etc.
here are some very good links you should know, I use them all the time.
http://www.clanwind.com/ffxi/fishing/ clan wind fishing database. Altho it hasn't been updated for a while, it's still very good.
http://www.titanictus.com/viewtopic.php?t=752 akirus' fishing guide. The best one you'll ever find.