This is a weight to volume conversion, very hard to do. Tablespoons are volume measures and equal about 15 milliliters, 2 tbs per gallon for ich treatment so that is 2x310=620 tbs. Then 620x15=9300 milliliter or 9.3 liters of loose, coarsely granulated aquarium salt. I don't know what that would weigh.
I just reread this post and found a problem.
I mispoke the treatment level required, it should be 2 teaspoons of salt per gallon, not 2 tablespoons.
This is important, if you have already added the salt at above level do a 2/3 water change now without adding more salt.
1 table spoon = 3 teaspoon
1 teaspoon = 5 milliliters
Therefore for 310 gallon the required amount is 3.1 liters NOT 9.3.
I am sorry about my carelessness in my earlier response.