When looking for salt, you want to find a brand which quickly dissolves in water. Also, some brands will use silicates as drying agents in their salt. As every reefer knows, silica in the salt will lead to red slime in the future. To avoid this, start with RO or RO/DI water and pick a good quality salt. Test with a silica test kit at each step in making your water so if you do have a problem, you know where it is coming from.