black stuff in toilet bowl-long [View all]

[irisblue]

I have 2 cats that like to drink from the toilet bowl, I keep the lid closed to keep the cats out. Interestingly the dog doesn't try to drink from the toilet....maybe bad memories from baths in the tub next to the toilet. I have had a problem with black stuff on the inside of the toilet, such that daily toilet cleaning wasn't keeping it clean looking any longer.There is no rust coming from any faucets in the house, so I figured it wasn't from the main water lines, just the internal pipes in the toilet. I did online research. I have a 6 yr old Kohler brand flap flush valve with a wash down type post flush rinse system. I drained the tank, blotted the inside dry then flushed the toilet and then removed as much water as I could from the tank, turned the water valve off and then cleaned the parts I could reach in this order 1-tank,2-small holes under the seat rim that runs water down over bowl sides and the lower,3- water level part of the bowl. The tank, did have some orange sediment in it, but no orange water was ever in the bowl; I cleaned the tank by wiping it out, then the pouring a half gallon of bleach, enough to cover the bottom and up the sides of the tank. let that sit for 20 minutes. I then opened the valve, refilled the tank, flushed the toilet twice, then redrained the tank and toilet bowl again and poured bleach into the bowl, I soaked some paper towel with bleach and laid them over the small water holes under the seat rim, just enough bleach so it would adhere to that area, waited another 20 min, came back, removed the paper towel and re filled and flushed the toilet twice. Redrained the tank and bowl again and poured in white vinegar in the tank, waited (you guessed it) 20 min, flushed the through the bowl, placed vinegar soaked paper towel under the seat again and waited...(good thing I only had one cup of coffee this am).
I reason that there must be some type of build up in the internal pipes in the toilet, while there is some orange sediment in the tank, there is no orange water in the bowl, just black. So could the bowl pipes be clogging with rust from the tank? Should I routinely run vinger thru the tank and bowl to keep the black stuff down? There is no way to stop the cats from drinking that water....I already have 4 water bowls in the house. Thanks for any help.