What would cause black slimy stuff to grow in my husband's toilet. I can clean it, and it begins to grow back in a couple of days. We have a septic tank. The toilet in the other bathroom (farthest from septic tank) doesn't have this problem. Nothing I clean with seems to stop the algae? From coming back. The house is selling and I have to be out in a few days, and I need HELP figuring out what to do. Thank you.

Look in the cistern (the tank behind the toilet). Is there mold in there, too?

If so, pour in some bleach, let sit over night. Flush, then clean out toilet.

It might be in there, and you keep re-seeding it.

Let us know if this helps.