The normal aquarium type API test is not sophisticated enough to tell the difference between the two.
You would probably need a much more expensive test kit, or a spectrophotometer and another battery of reagents to separate the two.
The kind of equipment I used to use in the lab, and cost more than my truck.
You could set up the tank, and add a sacrificial fish or two if paranoid, (its what I would do, if I had doubts).
What is the normal composition of your tap water?
If your tap water is hard and alkaline, it may be somewhat pointless to use it as an additive to the tank.
In my tank ph is near 8, and water alkaline, and seems to contain a lot of iron, but with the species of plants used (collected here), I can see the results of photosynthesis on a daily basis anyway.
When the sun is intense, thousands of tiny micro-bubbles are produced on plant leaves during that time.
I have tried to get photos but not been successful as of yet. I don't use soil, or ferts, but I hardly ever vac the substrate, and plant growth is lush. I guess it depends on the plant species, and if they are suited to water, and light conditions.