With afk fishing i don't think anything raises the actual chance at what you get, just the chance of actually getting something instead of failing.
Some people get good stuff at rank F while high ranked people get junk (followed by being pissed off that they wasted AP).
I went from rank F to A just AFK fishing, and never got a single item that could be sellable to a player, while rank F's around me doing manual were getting equipment on a regular basis.
So i'd say just leave it at F and hope for good luck o.o
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