Quinn Gehler, welcome to TFF. Reading this thread I think there may be some confusion so I will explain a bit further, all of which will agree with what essjay has beeen saying just so you know.
First, GH (general or total hardness) is the level of dissolved mineral in the source water, source water meaning your tap water that you will be using which is what most of us do. This is critical to fish, as some have a certain degree of tolerance but many have a very limited tolerance for GH outside their preferred range. And preferred range refers to the GH in which the fish will be healthy with no additional stress from inappropriate GH. Stress weakens the immune system, and brings on other issues and a shorter than normal lifespan. Freshwater fish have evolved to function in a very specific environment, and this includes the water parameters (GH, pH and temperature for our purposes).
We've no idea what "lower than usual" might mean, so we must have the number. The store should give you the number, and their unit of measurement (ppm, mg/l, degrees, etc), unless they are trying to hide something. It would be best to contact your municipal water authority for this data, perhaps on their website, or call them. Just make sure you get a number and the unit of measure they use.
Harding the water is going down a very involved road, and something you shouldnot even consider doing until you know the actual GH and pH, and then look at suitable fish. Every aquarium should have a regular (once each week) partial water change of at least half the tank volume, but a bit more is even better. Having to prepare special water in advance complicates things for you, and could mean death to the fish if an emergency occurred and you did not have water on hand.
Related to GH is the pH, and you can easily buy a test kit for pH. The GH will not generally alter in the aquarium [it would complicate this too much to explain how it might] but pH can vary and testing it regularly at least for the first few months is a good idea. It can indicate problems if it starts varying.
It really would be extremely irresponsible for any of us here to suggest possible fish species without knowing the GH and pH. Fish have a unique relationship to their aquatic environment, and they cannot just accept this or that if it is contrary to what they are designed by nature to need.
