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The rock was probably limestone. Limestone will increase the kh of the tank. The vinegar test is used for all types of rock to ensure that it is not loaded with any harmful parasites and/or metals.
You can also put the rocks in a bucket of dechlorinated water. Check after a few days and see if the parameters change. That way you don't have to guess about how much the vinegar is bubbling. To kill parasites etc. either boil the rocks or soak them in a bleach and water solution and scrub well.