This outlines why I hate fish. I have the exact same experience whenever I've ridden a fish - spin out on bottom turns whenever the waves start to get to a nice size

. I just put it down to my surfing style being incompatible with fish, I like to do big drawn out bottom turns then smash the fk out of the lip

.
I think with fish you have to stay up on the wall more and just wiggle about a bit

. I've spent a fair bit of time on fish too as I have a 6.0 I'm now selling.
I suppose when looking at the shape of fish its not surprising they ride the way they do. Basically they are short so they are like having 2 of the short rails from an asymetric - which is the top turn side, not the bottom turn side - hence they are rubbish at bottom turns! Opposite of a gun, which is 2 long rails of an asymetric - which is obviously the bottom turn side so they are not ideal for tight snaps in the pocket!
I mostly ride a 6.0 roundtail which is also harder to do drawn out bottom turns but still easier than a fish, and my other board (6.5 roundtail) is so perfect for these turns its unreal.
I really don't get the massive popularity that fish have gained over the last few years - they are fine in certain conditions but they most definitely are not a board for all conditions which is the way they seem to be used/marketed these days

.