The Kohler toilets we got in 2021 are bottom of the line. They do not have a strong flush and therefore can leave some waste behind in the bowl which is not good for keeping it clean.
We have a Nova whole house filter and also their water softener and we get the black stuff even though our toilets are frequently used and cleaned.
I have used the pumice stone and it works great for removing rings.
There are toilet bowl products where the bleach sits in a container inside the tank and some is released with each flush, so I believe that would avoid damaging inside the tank. Kohler makes self cleaning toilets where the flush is stronger and it has an enclosed area in the tank to put in a cleaner - that's our next step eventually. It's also smooth on the outside base making it easier to clean there as well.

What is the brand name of the tabs that do not corrode the seal in the tank? In 2012, we used 2000 flushes in our new TV house, which we had been using since the 1980s in four different houses--PA, NJ, MI well, MI city). In less than a year, the seals had corroded and leaked. Can't use 2000 flushes anymore.

I'd be very interested to know a brand that does not corrode the seal.

As I mentioned in my prior post, it may depend on the type of toilet. I HAD used the chlorine drop in discs for years in our prior home that had toilets with the "old style" rubber flappers with no issues.

In our current home we have the Champion 4 toilets that use a different flush mechanism that has a round rubber (or plastic/nylon?) ring seal fixed by a hard plastic cap that sits over the water discharge. In our new home I used the chlorine discs for a year and every month or two the toilets would develop a slow leak and I would have to change out the ring seals to get them to stop, only to have the leak reappear a month or two later. After reading up on the issue I found the info about NOT using chlorine in the water tank due to the effect on the seals. I stopped using the discs and since have NOT had the slow leak issue. But I guess your mileage may vary...

This seems to be a very long thread for something so basic. Toilet rings can occur(edit) anywhere you live regardless of your utility provider.

It only takes a few seconds to get rid of the black ring that forms at the waterline. Simply use Lysol Power Toilet Bown Cleaner. It has Hydrochloric acid.

If there is reason not to use it then please respond.
I do agree that the pumice sticks work very well but just take a little longer. Maybe difficult for folks with arthritis.

Whole house filter
 

You need a better whole house filter system.. I'm talking at least a $2000 plus. A system you can actually feel comfortable drinking your faucet water. That eliminates all minerals coming into your home. Cheaper systems just don't get the job done. And of course add a water softener to it. The best, healthiest, and why is this investment you can make on your home. Bar none!
I put in a Rain Soft top of the line system in, and I don't buy bottled water anymore. There is no doubt I have the purest, softest water in all the villages. Open your wallet and do what's right for your family.

Does a diabetic use this toilet? I am a diabetic and use the guest toilet and it gets this ring. My wife uses the master bathroom toilet and it does not get the same ring.

Not in Fort Lauderdale
 

I lived my whole life in Fort Lauderdale and never had these rings and had no filter system. Here it seems like it. The toilet isn't flushed every day there's a ring and I don't know what the heck it is.

This product may help?
https://youtu.be/lzXt-QB8Vzs?si=iiLk4hIAJ1y4RdbU

@Risuli speaks truth. No problem using chlorine discs in normal flapper toilets, don't use them in your Champion 4 (best toilet in the world IMO). It is the ring seal that will wear in that unit prematurely. I speak from experience and a past love of the chlorine disc replaced by a greater love of the Champion 4 flush.