The "black stuff" adhering to a toilet tank, whether inside or outside of the tank, comes from mineral deposits, stains or black mould. Mineral deposits are a result of high amounts of minerals in the water or certain minerals reacting with chemicals in the water. Black mould occurs when high humidity and heat in a confined area cause a rapid increase in the mould's life cycle. Because mineral deposits can damage toilet parts and black mould can damage a person's health, removal and preventive maintenance are essential.
Prepare homemade cleaners. Although commercial cleaners or bleach can remove black growth, accumulation or stains quickly, a harsh chemical cleaner can also damage the inside of your water tank---specifically rubber gaskets and plastic pieces. Several kitchen or general cleaning items also remove mineral deposits, stains or black mould: white vinegar, hydrogen peroxide, tea tree oil, baking soda and borax. Prepare these items as follows:
- Mold killer/mineral deposit remover: Fill a spray bottle with white vinegar.
- Disinfectant spray: Fill a spray bottle with over-the-counter (3 per cent solution) hydrogen peroxide.
- Sterilising antifungal spray: Pour 2 tsp of tea tree oil into a medium-sized water bottle and dilute with at least 2 cups of warm tap water.
- All-purpose cleaning paste: Mix equal parts baking soda and water, borax and water or borax, baking soda and water.
Shut off the toilet's water valve and flush the toilet to empty the tank. Carefully remove the tank lid and set it aside on a soft piece of carpet or towel.
Spray the mould killer/mineral deposit remover on all surfaces. Saturate any blackened areas except the gaskets. Wait 10 minutes, refill the tank and flush the toilet. If black marks remain, shut off the water, empty the tank again and follow the instructions outlined in Step 6. If the acid in the vinegar broke down the black stuff, flush the toilet two to three more times to remove any vinegar remaining in the tank.
Apply the sterilising antifungal solution to any remaining black stuff, wait 30 minutes and rinse. If any stuff remains, refill and empty the tank and then scrub the areas with your cleaning paste(s) and a scratch-resistant scrubbing pad or soft-bristled cleaning brush. Refill the tank and flush the toilet to rinse.
Empty the tank one last time. Spray disinfectant on all surfaces, wait 10 minutes, refill the tank and flush once.
Place a towel under the tank and spray your antifungal solution on all outside surfaces including the wall facing the tank. Wait 10 minutes. Wipe down the outside of the tank with a microfiber cloth or sponge. If black stuff remains, try the vinegar, cleaning paste and/or a melamine foam eraser---in that order. Wipe away the cleaners with a damp microfiber cloth between attempts. When finished, wipe and rinse one last time, spray with your hydrogen peroxide and dry completely.
Invest in a dehumidifier to reduce mould growth in your bathroom.
Always wear a mask (NIOSH-approved or small particle filtration), as mould spores can become airborne during cleaning. Always spot-test if you're using a cleaning eraser as melamine can scratch certain surfaces. Always check for cracks and repair with a finish repair kit as black stuff can easily adhere, grow and/or accumulate on raw, unfinished surfaces. Never use a cleaning eraser inside a toilet tank as rough surface interiors can shred the eraser into pieces that can cause water flow disruption or other damage.