Here's why:
* Osmoregulation: Blue Tangs have a delicate internal balance of salts and water. Saltwater tanks provide the correct salinity (saltiness) for them to maintain this balance.
* Water Chemistry: Their bodies need specific levels of calcium, magnesium, and other elements found in seawater. Fresh water lacks these essential components.
* Habitat: Blue Tangs evolved to live in coral reefs, which are saltwater environments. Fresh water would be a completely foreign environment for them, leading to stress and potentially death.
It's extremely important to note: Keeping saltwater fish is a serious commitment. It requires careful water parameter control, proper filtration, and a specialized diet. If you are considering getting a Blue Tang, make sure you are fully prepared to provide them with the proper care.