You need to remove 1000 ppb from 18,000 gallons and for their product one quart (32 fluid ounces) removes 10,000 ppb from 10,000 gallons so you need (32 fluid ounces) * (1000 ppb / 10,000 ppb) * (18,000 gallons / 10,000 gallons) = 5.76 fluid ounces, NOT one ounce. Their pint has 16 fluid ounces so has more than you need, but it's not extraordinary and I don't think you'll find a less expensive bottle of another product.