According to this Islamic ruling and fatwa, all dead sea animals are halal. It does not matter if the fish died naturally or due to some external cause. So, if fish die because of chemicals in the water, they are still allowed to eat.

However, Islamic scholars also say that if eating such fish may cause harm or illness, then it becomes haram due to the risk to health.

This fatwa is based on the opinion of many Islamic scholars, who state that fish and similar sea creatures are permissible no matter how they die, unless they become harmful to eat.