carassius auratus, crucian carp, surf perch
鯽 centers on a specific group of fish: crucian carp, goldfish (Carassius auratus), and surf perch. It is a zoological name rather than a broad everyday concept.
鯽 combines 魚 (fish) with 即, which likely contributes the sound. The character was created to name specific fish species.
The left side 魚 means fish, and the right side 即 suggests something immediate or close. Picture a fish that is immediately recognizable as a crucian carp or surf perch when you see it.
For セキ, imagine a fisherman shouting 'Seek!' as he looks for a crucian carp: seek -> セキ, and the fish 魚 is the one he seeks.