what happened to arlong after arlong park

Arlong was defeated by Luffy, and the Marines later arrested the remaining members of his crew; the strongest summary available here suggests Arlong himself was captured and sent to Impel Down, then later freed by a pardon tied to Jinbe becoming a Warlord, after which he returned to his old East Blue ambitions before being beaten at Arlong Park.

What happened after Arlong Park

• Arlong lost the battle at Arlong Park when Luffy destroyed the park and defeated him.

• The Marines arrested the rest of the Arlong Pirates afterward, while Hatchan was the notable exception who escaped capture.

• Another source says Arlong had previously been captured and sent to Impel Down, then received a pardon because of Jinbe’s status.

In plain terms

The story does not treat Arlong as dead; he is generally understood to have survived and been imprisoned or otherwise taken into custody after his defeat.

That’s why fans often describe his fate as “arrested, not killed”.

TL;DR

Arlong was beaten by Luffy, his park was destroyed, his crew was arrested, and Arlong himself was not killed.