RE: Fin Size Isonic 125
Definitely note Roger's comment about the SR6b version being faster.
On iS this is even more noticeable, as iS* is typically a low/er drag/higher efficency tuning combo than HS, which was tuned mostly with a high/er power with high/er drag combo.
Cheers ~ Ian
*Slalom sizes / such as would use the HS size fins 34/44 etc