OK, I'm three years late but am just seeing this now.
I'm no expert but here's my opinion on this. I would use 60 grit in a random orbit sander to remove the gel coat down to the fiberglass.
I wouldn't go any finer than 60 grit as I would then use Pond Armor to coat the entire inside and they recommend using 60 grit.
Pond Armor comes in several different colors and I prefer black over the ugly blue these tanks come in.
It isn't cheap but it works great and from my personnel experience with it, it holds up really well if the surface is prepared properly.
There's lots on information on this site about Pond Armor.
Thanks for reading.