Yes - definitely add them to the change water, rather than putting them directly into the aquarium. Particularily to prevent KH fluctuations, as they are a threat to plant and livestock health. You can add the GH mineralizer directly to the aquarium if need be, but beware that it will cloud the water for at least 15 minutes. I'd recommend keeping both KH and GH above 2 unless you have a buffering substrate (Ex: Controsoil). Beyond that, just take into consideration the recommendations for future livestock. If keeping shrimp, I'd recommend raising the GH to 6-8.