My guess is the CNN was trained on highly abstracted stereotypical-fish-drawings, not on actual pictures or high-quality drawings of fish. I put in my best effort to draw a good-looking fish (although I'm no artist) and I got 35%. Then I drew a basic single-stroke fish and got 65%.

Magikarp is not a fish to a robot

It's not a good fish to pokemon trainers either. 35% seems about right.

Try SolidGoldMagikarp

this piece de resistance swan song is only 35% fish but then my fish tank is like 90% various formats of blackface fish, nazi fish, and swimming penises