I just bought the Merk test last week, since I still had Algae. Now, at least, I know why! I have a very heavy Bio-load, so I raised the amount of ROWA in my rector, so I could raise the flow a bit. I am also feeding less. However, with Salifert, I was "at my wits end", since it was testing at zero, and I thought I was doing everything possible. My algae has stopped growing, but I know now that I still have a problem. Now I have the correct information to act upon.