How and when to dose iron.

Yellow rubber or yellow slime: It's not getting enough iron, compared to how much light it's getting. The more light it gets, the more iron it needs. The best way to give it more iron is to increase flow, which also delivers more of all the other nutrients to it; this will increase overall filtering too. But at some point your screen will not be able to hold on to the heavy algae. Or, maybe you cannot increase flow any more. In this case, add iron, such as Ken't Iron+Manganese, according to the instructions. This will help the algae grow green without slowing filtering. If you can't get iron, then turn the lights off an extra 2 hours per day. Don't move the lights farther away; just turn them off 2 more hours per day (weak light is not good; it's better to have strong light for a shorter time). This reduced lighing time will also allow the algae to grow green, but it does slow down the filtering because the amount of filtering is proportional to the amount of light.