Thick mineral stains at the bottom of only some of my toilets

[Brad Mathews]

We have two toilets, both in a 2010 addition that get thick brown/black mineral stains in the bowl, but mostly in the very bottom. It comes off with a razor blade.

The other three toilets in the old house do not have this problem.

The old house has mostly been upgraded to copper over the years with still a few pieces of galvi through the garage.

The addition splits off before it sees any of the galvi.

Any idea why just the two have the problem?

Thx, Brad

 

[Geofd]

Staining or collecting???? At the bottom
Are they the same brand, have a friend having the same issue he has a toto, no staining ,an eljer that's staining, I'm guessing in his case the eljer is a less quality brand, and older too I think it has to do with the way the toilet is prepped and painted I think the eljer prep and paint if you really looked at it was a little
Course to the feel and that would allow growth ,it's just a guess,he's gonna try
Emptying the tank and taping the holes on the rim and filling with vinegar if you Google toilet stains there are many remedies out there (my friends issue starts were the jet holes are and streak the sides)