The API test kits give you degrees of hardness, one degree for each drop of test reagent it takes to change the color. So, you have hard water, with 15 dKH and 28 dGH. The KH measures alkalinity, which is assumed to be from carbonates, and the GH measures the amount of calcium and magnesium, primarily, in the water. Your tank water is slightly lower in KH than the tap water, either from measurement error, or possibly because some plants in the tank are using the KH for a source of carbon.
Neither of those tests tells you anything about the iron in the water.
You do have high light intensity, which means you need to be using pressurized CO2, and a good fertilizing program.
You don't need to do anything about the hard water unless you are trying to grow some of the few plants that require softer water to grow, or are trying to breed some fish which also require softer water to breed. I think it would be a good idea to wait on the discus until you feel that you are comfortable with fertilizing, water change needs, growing plants and not algae, etc.