Where to buy cheap marine salt ?

[jhseah]

Asking on behalf of a friend, where to find the cheapest salt used for marine tank?

Currently, he is buying seawater from a lfs at $5 per 20Litres.

[etan]

There will be cheap n ex brands of marine salt depending on quality. A reasonable brand I m using is HW Marinemix Professsional. One bag cost $12/, can mix up to 150L.Can buy at most marine LFS, I think,I bought mine at Keong Seong

[jhseah]

Thks etan. The marine salt from US or Germany?

Can anyone explain to me why must use this special marine salt? What's the difference btw marine salt and the normal salt added to freshwater tanks?