Old, animated movie, where fish in the ocean fight versus poison crates dropped by "evil men."
One scene I remember is where two fishes eat the poison, and then they shrivel and die.
The story revolves around a group of ocean fishes where they suddenly find themselves threatened by pollution made by men. Specifically, I remember crates with skull signs on them falling on the bottom, cracking on one corner and spilling the black poison. Then some random fish started eating it and after they have eaten it, then shrivelled and died. There was a male and a female fish protagonist (maybe sturgeons, 10% sure). Some fish ate the poison later in the goal to save the protagonists.
Animated movie was old, depressing etc. Cannot remember more because I watched it around 2003.