Aquarists spend a lot of effort trying to get the salinity levels in their marine aquariums just right. And if they don’t they really should since fish have evolved special mechanisms to regulate the content of water inside their body in a process called osmoregulation.
The question of ‘Do fish drink water’ might seem silly at first listen, especially for an animal that spends all of its life submerged in the stuff, but it turns out that freshwater and saltwater fish differ massively in how they osmoregulate. Freshwater fish are constantly absorbing water from the environment, while saltwater fish are constantly losing it.
In saltwater fish particularly, the process of osmoregulation takes a lot of energy for these species which is why we sometimes prescribe keeping some species in low or hyposaline conditions to help with conditioning, to fight off disease or simply to save money on salt. When you consider how specifically fresh and saltwater fish manage the amount of water in their tissues, you’ll better appreciate the need to give fish an appropriate level of salt in their aquarium water, whatever that may be.