Additional bottled water is generated when fish die

Oh! That's why I always found some small bottled water inside my fish tanks.

It is seems like a bug, but it is a feature. Gulp fish transforms polluted water into normal water. Most probably it work by portions - like, 10kg polluted water in, 10kg water out. In case if your fish consume 12 kg of polluter water, it will transforms and execrate 10 kg of water, and keep 2 kg inside as a container. In case if fish die, container destroyed and water going out "bottled". Same behavior happens if you destroy any other liquid container - water tank, liquid sieve, desalinator or aquatuner. 

PS for sure it is a bug, fish is not smart enough to pack water into bottles unless it is mutated dolphin under cover.