Fish get up and go quickly because of their planing efficiency (width and low rocker). You just don’t need a lot of
bouyancy to make them go. Once you’re at speed, excessive bouyancy hinders maneuverability and gets skittery.
If you want to just catch waves and go straight, thick is good. If you want to turn, however…
Reverb posted some pics a while back showing just how thin you can go on the tails. What I meant by my ''thin as
you dare’’ comment was to encourage DIY’ers to be a little brave. Get foam out of the crack aggressively, and then
use the stringer curve there as the guide for the rest of the inside of the swallow. And this is all relative to the rider’s
weight, it’s OK to leave a little more on a big guy’s board.
I was curious to see your comments on the thin tail affecting wave catching ability and your comments on planing ability make sense.
In going thin in the tail like that, is there an ideal glassing schedule on poly to give a good strength to flex ratio?
the thread you mentioned is here i believe
http://www.swaylocks.com/forum/gforum.cgi?post=369012;search_string=reverb%20fish%20tail;#369012
Good question on poly lam schedules to reinforce/stiffen those thin tips. I should have mentioned that
a butterfly tail patch on the bottom strengthens the tips and the fin area. This is a common practice in
the quality fish biz.
Thank you so much for those very detailled answers, for sharing your knowledge.
I really got the angles by reading this thread.
I will work on my design and try to have a thin tail and leave those boxy/medium rails aside.
My starting point was the Knistle or Lis fish design as described in the surfboard design construction 1977 manual and when you have a look at it the rails aren’t that boxy 
but when you go to surf shops you see those fat rails.
Your answers really helped me a lot to refine my vision of the fish.