Natural sea shells are mainly calcium carbonate with a small amount of magensium carbonate. If there is free caroline in the water with will react with the calcium carbonate creating Calcium chloride and CO2. This also pushes up GH. Calcium chloride is safe for animals and plants and it is often used in commercial GH booster. Natural sea shells will neutralize chloride and excess sulfates that can push the PH down. Most fertilizers and GH boosters have more sulfate than needed so after the plant consumes the GH excess sulfate remains in the water and will cause the PH to drift down. Calcium carbonate is only soluble in acidic water. It will not dissolve in water with a PH of 7 or higher I use a GH booster and use seashellsI to keep my PH near 7. I don't know what they use to make wonder shells but if it has calcium carbonate it would work like a regular sea shell. Crushed coral is also calcium carbonate.