If you're anywhere southeast of London, chances are good you've got calcium carbonate underground, so your readings aren't surprising to me. You can use Reef Advantage Magnesium to dose specifically for Mg and Flourish Potassium for the K. This won't increase your GH nearly as much since the Mg is needed in much lower concentrations than Ca (approx. 5 ppm Mg vs approx. 15 ppm Ca)
Kind regards,
PE
Kind regards,
PE
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