Hi peeps, i normally do my water testing over the wknds,but for some i reason i decided to test it tuesday.Bear in mind i tested the water the sunday before. To my big surprise my po4 was out of this world high,instead of test kit reading very light blue or clear,the tested water turned navy blue!! I had no phosguard in the tank so nothing to drop that po4's or keep it at bay. Then a fellow reefer gave me some phosbuster,from caribsea. Damn that stuff works kwiktreated the tank the same nite,the following evening my readings were ZERO!!(did the test twice,2 different test kits) So i would say defnitly invest in this product. between the sunday and tuesday,all i did was top up with about 13 litres of purchased RO water,which means they selling me crap Ro water now i got myself some phosguard aswell. just thought i had to share my experience with the product.