Water testing kits available to the aquarist are very basic. You could compare them to the home chemistry kits for kids as opposed to a professional laboratory. You, I, and most aquarists can't test for the bacterial levels in aquarium water, large frequent water changes are a way of assuring these are at as low a level as possible.
Generally bad bacteria are water borne, good are in the filter, as well as the bio coating that marks a mature tank.