Hi and welcome to Canreef.
As stated above Instant Ocean or Kent seem to be the most popular. Are you going to use tap water or RO water? Also if you don't already, get a good book on reef or marine fish keeping. That will help with some questions you will have later down the road and let you know what kind of fish can be kept in that size tank and how to care for them properly.
Enjoy the hobby and I warn you now...............................it is addicting
HTH