The high GH is not due to sea shells as there are none in the pond at present. I'm not a geologist soI cant say what the exact makeup of the water catchment is here. I do know however that the majority of the rock which forms the island is granite and other very hard rocks of that type. There are no significant chalk deposits. We get incredibly hard limescale buildup in kettles etc, so the pond GH being high is not too surprising. Also the lack of limestone or chalk might explain the low KH value. I will try a test on the tap water as well as a re-test on the pond. If necessary I will do a part water change and add baking soda. By the way, is baking soda the same as sodium bicarbonate?