It depends actually. If their tendrils are akin to the jellyfish they're based on them, instinctual aversion would divert many fish away from them, unless they themselves were seeking food.

If memory serves, a magikarp, while may not being the sharpest knife in the drawer, are incredibly hardy and blessed with armor-like scales. To that end, it's possible that a magikarp could swimp amongst them unimpeded.

I anticipate perhaps more-so with the assistance of a baited hook (image not withstanding), it would be a remoraid or one of those fish from X/Y. Baskin I think?