I have a dragon goby and I need to set up a brackish water tank for him. I just bought some Instant Ocean Sea Salt. I've looked all over for what the specific gravity should be and how much salt to add, but I have seen some very different information from 1 tbsp/gallon to 1 cup/gallon. Also, how long should it take for the salt to completely dissolve in the water so I can check the specific gravity? Thanks.