Im not sure if I am posting in the right place but I have some questions about cleaning coolant tanks. Recently I cleaned 2 tanks at my job. They were neglected for decades. Tons of mold and bacteria. Anyways I cleaned both the same way. I took out the coolant, cleaned the tank, ran cleanall through the system and then put fresh coolant in. Well it appears I didnt do a good enough job on one of them as there is black film on top of the coolant. I dont want to go through the whole process again so is there an easier way to do it? Even if its just temporary. I plan on getting oil absorbent paper towels and skimming the nasty stuff off of the top but does anyone have any suggestions for some type of additive I can put in the coolant to help keep it clean? Any other ideas that arent time consuming?