The bottles have pretty detailed directions on the back. I would start off with the recommended dosage which I believe is 1 teaspoon per 20 gallons a day and go from there. After a few days you can start testing and see if you need to increase, but you're levels aren't horrible so you shouldn't need too much. I've never tried the gallon size, I like to stick with the small pre-mixed bottles, so I honestly don't know how far a gallon would go.