I had need to remove a radiator for decorating and decided to top up the system with a corrosion inhibitor when I replaced the radiator and refilled. The header tank was liberally coated on the sides and bottom with a thick brown slime, likely some organic growth I guess. Is growth in these tanks normal and how often should they be cleaned? I would guess this tank has not been looked at for the past ten years or so as is likely common with most tanks.