The "2 drops per gallon" dosage is erroneous. Marine Depot in the US received a number of complaints from customers indicating the product was not working. The manufacturer changed the dosage level up to 4-drops per 10 gallons:
http://209.85.141.104/search?q=cache...n&ct=clnk&cd=1
Note that this is the cached version of the page. Looks like Marine Depot has pulled the product probably because of those complaints.
Interestingly, the box that it came in says "4 drops", but the bottle itself says "2 drops". Regardless, from my experience the dosage should be "at least 4 drops per 10 gallons".