It's not the KH of the water that constrains the type of fish you can have but the GH.
GH is general hardness but despite that name it only actually measures divalent metal ions. For our purposes this is calcium, with maybe a bit of magnesium. Some bodies of water have only small amounts of calcium and fish in it have evolved to cope with that level; other bodies of water have a lot of calcium, and fish there have evolved to cope with a lot of calcium. Some fish can live in a wide range of GH, some need specific levels; rams need soft water (low GH) and Rift Lake cichlids need very hard water (high GH).
It is the GH figure, usually called just hardness, that you need to look at when reading fish profiles.
pH is connected with hardness but not directly. The type of minerals in the water cause the pH value. It is theoretically possible to have hard water and low pH, though that rarely occurs in nature. I have softish water and pH 7.5. pH does affect fish too but not as much as hardness. So I can keep fish that prefer soft acid water more easily than those that prefer hard alkaline water.
KH comes into play in stabilising the pH. It doesn't affect fish directly, just by stopping pH swings which are not good for fish. And of course low KH affects cycling as the filter bacteria need carbonate in the water to multiply quickly.
I've just gone back through your posts and see that your water company says your GH is 2.8 german deg/50ppm. That is classed as very soft. Unless you add something, you will be restricted to fish from soft water origins such as the Amazon. Crushed coral will raise things a bit, but not much as it is not very soluble. I did an experiment a while ago, where I put some crushed coral into a tub of water and left it for a week. I also left a tub of plain water alongside as a control. After a week, my pH had risen by 0.2; GH by 2 german deg and KH by 2 german deg. As you can see, not a very big effect. You might be better off with remineralisation salts if you want to keep fish from anything other than soft water regions. The amount you'd need to add would depend on just how hard your water needs to be for the fish you want to keep. if you stick to the softish end of the spectrum, you wouldn't need very much. But if you want to keep Rift Lake cichlids, you would need to add a lot.
Look for Kent RO right, and Tropic Marin remineralisations salts - the tropical one not the marine one.