Welcome to Tropical Fish Keeping forum.
On the GH and KH, just contact your municipal water supply folks; they likely have a website, and this information may be posted. Or if not, they can tell you. Knowing the GH is important, and the KH too as it will tell us if the pH is likely to lower or not. Some fish have very specific requirements in GH and pH, as the profiles will point out.
Glass Catfish is one, as the profile will tell you. As for the Clown Loach, my advice is to return these now before they get settled and develop problems. Fish grow continually, and this species will attain 12-18 inches, but when they are growing in too small a space, they do not develop properly. Internally they can be stunted, or at the very least stressed which causes other health problems. Much better to return them now, as later may be too late for them.
Your light is fine for plants. Below is an old photo of my 29g with this same single T8 Life-Glo tube. The plants are all low and moderate light requiring, they are in our profiles too.