Howdy Sheester, What is the water temperature in the reservoir and is it well aerated? I use ice bottles to keep the temperature between 63° F and 79° F and run four 4 inch by 3 inch air stones on a quad outlet air pump for each of my reservoirs, Algae, molds, and slime can grow in warm water (72° F and above) and can cause all sorts of problems. I have been using GH FloraShield (a cleaning wash) at a low rate, 1.5 to 2 ml/gal, to keep down unwanted organisms in the reservoirs. This, with the temperature control, has worked well for me. One other thing that You could try is a good dose of hydrogen peroxide (the 3% ), I have used it at 4 ounces in my six gallon reservoirs). I'm not familiar with the plagron pure zyme, and if it is similar to the Hydro-guard, the H2O2 will kill it as well as the unwanted stuff.