Not sure if this is completely correct but hard water is what would come out of a well/ ground, where soft water usually has salt in it. Usually how it is referred around our house. If it affects the ph I don't know as some days our water is hard and other days it is soft depending weather or not we put salt in or not.
I was always told it had something to do with the minerals or whatever in the water. Meaning hard water had more of certain minerals. I know, not the best information/description. There are different websites about it on google.
Hard water is, I am pretty sure, OS having a higher mineral concentration. That is why you would get hard water stains. My water is on the soft side but I still have a high pH, it all depends where it is from and what they add to the water. Usually a high pH means harder water vice versa, but it all depends what your water company adds.
I found my parameters on my water company's website, they give you the numbers so you would have to find a site that tells you if you have soft or hard water based on those parameters.
It's interesting that this keeps coming up when it has little to do with betta fish; this is a rather advanced fishkeeping topic. Hard water is water with a high mineral content (like bottled spring water). Soft water has a very low mineral content (like reverse osmosis [RO] water). There are two components to water "hardness" GH and KH. KH directly impacts pH (which is why you can't fiddle with pH if you don't know your water hardness) but does little to fish. GH doesn't affect pH but has a large impact on the fish. Because the fish and the water are so closely intertwined, over millions of years individual species have evolved to bring their blood pH and mineral content in the body very close to what is in the water. This is so they don't have to work very hard to keep minerals out or in. But this also means when we keep them in water parameters different from what their body is used to, they have to work harder to keep their internal balance and keep their organs working correctly. Kidneys are the first that come to mind because they play a key role in osmoregulation; one of their jobs is to filter out any extra minerals in the blood. And when a fish is evolved to live in low mineral content water, existing in high mineral content water is going to put a lot more minerals in the body via diffusion over the gills than the kidneys are used to handling. This is one of the reasons we don't recommend soft water fish in hard water and vice versa. In soft water, hard water fish like many African cichlids and goldfish (even though they have been domesticated for around 1000 years) will be starved for minerals in the body.
But what is important for everyone here to know is that betta like stable conditions. It doesn't matter so much what your tap water pH or hardness is, it matters that those parameters stay stable in your fish's tank. Regular water changes are the best way to do this.
I'm trying to look this up and such right now, but when using the tap water at my gf's apartment, our betta practically died, at first we thought the cats were bothering it so we simply moved it to a different room ect. but then he got much worse. so i took him to my house, different water, tank the works. he made a full recovery and doing fine. So, i brought him back to the gf's apartment, in the same place, same tank, just using water we bought at walmart. so i'm convinced it was something in the tap water here. is there anything particular that i should look out for on the websites?
Still don't get why bettas for some reason dont care what pH or hardness they are in but other fish do...
Actually they do care to a degree. But like some of the tetra (glowlight tetra come to mind) they have a very wide range of tolerances. SeriouslyFish states it as between 5dGH and 25dGH, and Fishbase.org puts their hardness range between 5dGH and 19dGH. What this means is that you either need very soft water or very hard water to cause problems for betta. I have actually seen people having trouble with betta in very hard (basically liquid rock) water.
EDIT: Ringo, I think you will have better luck and get more responses if you start your own thread and give some more information such as tank size, temperature, water additives, ect. Oh and welcome to the forum!