For the many updates, check label "toilet"
I'm talking about:
Kaboom Scrub Free!
Fluidmaster Flush 'N Sparkle
I stumbled upon the earliest form of toilet tank mounted cleaner on the net many years ago. The idea is so good that I brought it. It looked a little more complicated as there seemed to be a miniature valves into and out of the cleaning tablet housing. I was let to believe that the valves were the important part, controlling the correct dosage of the chemical.
Everything was good except that the chemical didn't last. I thought I had the idea of waiting for one of the giants to enter the market and they can produce some decent chemicals. I actually asked the website about plumbing code, whether it's safe without the possibility of back flow. I think the toilet valve itself guarantees that. The website soon disappeared and I thought it folded.
I think Kaboom is the next to pick up the idea. It was well made and the hardware was free too. I think I still have the early version called No Scrub somewhere under the sink. It used crystals and was still dissolving too fast. I don't think it last 3 months. The housing is a bit big and I thought of putting something else in it too but if I never thought I can come up with something better than them. The lid is screwed on and it's a bit hard to open it to refill.
Than came the Fluidmaster Flush 'N Sparkle. It's more professional. It's compact. The cartridge is easier to pull out and replace, because there's an O ring to seal the housing, requiring less effort than just a screw cap. The major advantage is that it's available in hardware stores together with other Fluidmaster hardware. Again it doesn't seem to last 3 months or 1000 flushes, but long enough.
Yes, all the stuff works well. You don't really need to scrub. Try the Clorox tablets, 3" white bleaching blocks in supermarkets and everywhere. You drop the thing into the tank and that's it. The problem is that you can't control the dose. In the morning the first flush is concentrated chlorinated water. It's worse if your toilet has no windows, relying on extraction fans for the ventilation. The other problem is that chlorine attacks the soft rubber flapper of the flush valve. I think you have to replace them, say, every year or two. Consequently there are chlorine/ chemical resistance versions in the hardware stores. It's trivial to replace the flappers, but it cost a few dollars, and otherwise those things never break.
The only thing you need is chlorine tablets like Clorox. It's simple chemistry. Hard water contains mineral which are calcium minerals, or from natural lime stones. To dissolve them you need acid, and chlorine react with water to form hydrochloric acid. I hate corporations trying to sell bacteria killing stuff to ignorance housewives. Bacteria cannot survive without water and air. And if chemicals can kill bacteria they can also kill you in sufficient amounts. This time chlorine tablets are over kill for killing bacteria. In this case, you really don't need to kill bacteria in the toilet. There's plenty of them and they don't come back. Bye. The bad thing is that they will kill the good bacteria in the septic system.
Other cleaners without bleach are just like detergents, surface agents preventing dirt from sticking on the surface of the toilet bowl. They don't work that well against lime and definitely not on lime filled toilets. With chlorine tablets old toilets will become sparkling white with time, if the surfaces aren't damaged by scrubbing. The other acid cleaner are CLR, now with an organic acid form. It's as good as any toilet bowl cleaners. But I can't find any slowing dissolving acid in tablet form.
As for environmental impact, chlorine isn't that harmful. A small amount isn't toxic as it's used in most swimming pools and in the water. Chlorine will eventually form natural occurring stable mineral salts, which is totally harmless. As long as you can use as little as possible, you don't waste resources to produce it and minimize changing the acidity of the discharge environment.
The Fluidmaster is good but I have 3 toilets. I have to replace them all in one to two months. Sometimes I was too lazy to do it and it cost about $8 each? Using a brush and a liquid bleach every few months isn't a big deal, until one toilet turned black at one of the surface cracks. Well, the option is to replace the toilet that isn't necessary and too big a deal if I were to do it myself. If you keep it sparkling white you can't see the cracks.
The story goes: one day I found an old Clorox tablet 3" in diameter. I found out that the Fluidmaster cartridge can be opened to refill. So I crushed the tablet into smaller pieces and put them back in the cartridge.
I searched the net and if you have a swimming pool, you know what I was thinking. I found those 1" diameter chlorine blocks for swimming pools. Buy many swimming pool owners have thought of it and tried it. One plumber guy used it and do not recommend it mainly because the tablets are of different dissolving rates and the swimming pool is a lot of water compared to the toilet tank. But he was dropping the blocks into the tank like the Clorox ones.
I think those swimming pool chlorine tablets will work well. Firstly they are slow dissolving. They are much cheaper than those for toilets. The 1" ones fit right into the Fluidmaster cartridge that I measured. If these tablets dissolve too fast and you loose money compared to using the original cartridges, you can leave some time gaps between replacing the tablets. Acid is rather good in dissolving old calcium deposits. You just need to replace the tablets before deposits become visible, or just become visible.
Further down the road I don't think you even need the dispenser. It's pretty cheap to make one perhaps. If not you can just make one to show off. Parts:
1.5" diameter PVC tube (enough to accept 1" tablets)
John guest adapters on both ends, or tube adapters
two sort tubes
The tube used in the toilet valve seems to be not the standard 1/4" tubes, but at least they are standard. You must be able to find it in the hardware stores or cut the original piece into two. The installation becomes inserting the outlet tube into the central tube of the tank, securing using fancy clamps or just rubber bands. You can mount the whole thing at the side of the tank of just drop the sealed tube into the water.
You can have a fancy union to make it easy to open the tube and refill tablets. Really there are plenty of space in the tank to do some awesome stuff.