If a tank has be experiencing high phosphate levels it is possible for substrates (sand, rock etc.) in the tank to lock in this available phosphate. As the Rowaphos brings down the ambient levels in the aquarium water this will be released out of the substrates giving the impression that less phosphate has been removed than is actually the case. In these situations it is best to continue to use the reccomended ammount of Rowaphos and change it more often, every 2-3 weeks, this will get on top of the issue and you will start to see the phosphate levels decrease to desirable levels.

During this time it is also best to check that the tank is not being overfed as this will compound any phosphate issues.