eel
鱔 centers on the eel, a long fish-like creature. The meaning is specific to this animal.
鱔 combines 魚 (fish) with 善, which likely contributes the sound セン. The character was created to represent the eel, a type of fish.
The left side 魚 means fish, and the right side 善 suggests something good. Picture a delicious grilled eel, a prized fish dish.
For セン, imagine a hundred (百, ひゃく, but here use 千, せん) eels wriggling together: 千 (せん) -> セン, and the eel is a fish.