Re:CO2 test kits

I have tested the following water samples with a Tetra CO2 test and got
identical results: water from an aquarium which has CO2 injected at a rate of
about one bubble (standard airline) per three seconds in a 55 gallon tank; a
bucket of java fern (no fish, dim light for a month); and Milwaukee City tap
water. All readings were at 15 ppm+. Should this be so? Are there any other
test kits you recommend? I have measured hardness with aquarium pharm kit, but
am not sure if it is measuring total hardness or DH. Can anyone help?