The black, oily film inside a toilet tank usually comes from one of two sources. The actual parts inside the tank that make up the flushing mechanisms could be one culprit. The problem might also be the water that refills the tank after each flush. You may be able to repair the problem with a simple cleaning. If not, you could be on your way to the hardware store for some replacement parts.
If you remove the tank lid and look at the components inside, you'll see that they are mostly made of plastic and rubber. These parts are the fill valve, usually on the left side inside the tank, and the flush valve, to which the rubber flapper attaches. When these parts begin to wear out and corrode, a black coating forms on the water. If you touch one of these corroding parts, such as a flapper, with your hand, you'll see the black film on your hand.
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Additional small parts within the tank that are not as easy to spot as the fill and flush valves are the rubber washers on the tank bottom. They might be hidden directly underneath the heads of the tank-to-bowl bolts that anchor the tank. When these washers start to fail over time, a black film forms inside the tank. Repair this by replacing the washers. Do it promptly, or the worn-out washers will allow the tank to leak around the tank-to-bowl bolts.
If the water coming into the toilet tank from the well or city is hard water, a buildup of mineral deposits could form inside the tank and the bowl. These deposits can clog the bowl's rim holes and impede the water transferring from the tank to the bowl during flushes. If you suspect this, have your well water tested for hardness, or learn the hardness of the city water, which you can sometimes find on the water company's website.
The black, oily coating inside the tank might also be mold. Moisture is a breeding ground for mold, which will grow if the tank is not cleaned on a regular basis. Be careful about using chlorine tablets or bleach to clean the tank because you might only end up damaging the flushing mechanisms. Turn the shut-off valve completely to the right to keep water from refilling the tank. Empty all the water from the tank by flushing the toilet and then vacuuming out the remaining water with a wet/dry vacuum, then thoroughly clean the tank's inner walls and floor.