The water that comes out of your tap must come from somewhere. This will be either down some mains pipes from a reservoir or river etc, or from a well. Your parents/guardians should be able to tell you which, because if it comes from a reservoir etc they will have to pay a water bill to a water provider, or if it is from a well they'll have to have the water quality checked every so often.
If they pay a bill, ask them the name of the company, or even the website address off a bill. Then go to that website and see if you can find their section on hardness. If you can't find it, tell us the name of the water company and we'll see if we can find it.
If your water comes from a well, your parents/guardians should have a water quality report, and that may or may not contain hardness info.
Natural water sources - rivers, lakes etc - have minerals dissolved in them. Some have lots of minerals (hard water), some have only a few minerals (soft water). The fish that live in these natural water sources have evolved so that their bodies can cope with that type of water. If you put hard water fish in soft water, or soft water fish in hard water, their bodies struggle to cope. The wrong type of a water stresses fish and stressed fish get sick and die easier.
Of the fish you have, mollies must have hard water - water with lots of minerals dissolved in it. Their bodies have evolved so that they can deal with all those minerals. If the water isn't hard enough mollies get sick very easily.
Neon tetras, hatchetfish, cories and bettas need soft water - water that has only a few minerals in it. In hard water they won't get sick as easily as mollies do in soft water, but all those minerals in hard water collect inside their bodies and they don't live as long as they should.
We should try to keep fish that come from water with the same hardness as our tap water. We should always research any fish species before we buy them. The best website for researching fish is
http://www.seriouslyfish.com/knowledge-base/ The fish profiles on there tell us how big the fish grows, what size tank they need to be in, what water hardness and pH they need, what temperature they need, what food they like, what kind of tank decorations they like, and so on.
For example:
neon tetras
http://www.seriouslyfish.com/species/paracheirodon-innesi/
bettas
http://www.seriouslyfish.com/species/betta-splendens/
I can't give links to the other fish because I don't know what type of cory, hatchetfish or mollies you have.