Not necessarily a genetic defect. Did your fish lose it's fin as a juvenile? Several of my Polli's dorsal finlets are crooked, and I'm quite sure it's because they were bitten off/severed as he was still very young and growing quickly. My theory is that the finlets have been confused as to how to put themselves together, so to speak. So it might actually be a direct result of their aggressive regeneration abilities.I had a sen that was missing several fins and never regrew them. Some people said they'd probably been bitten off, but I always believed it was a genetic defect and this has strengthened my opinion. Very interesting!