I'm assuming you mean Calcium Carbonate (measured as KH). High KH levels won't harm your fish as has been stated before, but may contribute to "limescale" build-up, and if you have soft water fish they may not breed successfully. The best removal method is using a RO/DI filter, but altering your water chemistry is NOT recommended unless you have a specific need for doing so and have a good understanding of what you are doing. RO filtered water is almost completely demineralized which means that you'll need to rebuffer it with either a commerical product, or by "cutting" it with tap water, because fish and plants need some mineral content in the water. Also, this means a long term commitment to always premixing your water, replacing filters, and securing an alternative soft water source if there is a problem with your unit. See why it's always recommended to just stick with what you have? ~_^