Water hardness (GH) is strictly a measure of calcium (Ca) and magnesium (Mg) in the water. All animals and plants need Ca and Mg to live. If you don't have any the plants and animalism the tank would die. Some do well in soft waterwheel others do best in harder water.
corals and limestone contains mostly calcium with some magnesium. Adding this to the tank will add some GH but in my experience it only increase GH by 1 or 2 degrees. How much of a increase you get will depend on your water chemistry. it alone would probably not b enough to reach a GH your fish will like). Themain reason it doesn't cause a larger GH increase is that mainly dissolves in acidic water. The lower the pH the faster it resolves. As it dissolve it pushes the PH up. Once the PH reaches 7 it stops dissolving
To increase your GH more you need calcium and magnesium materials that will resolve at all PH levels. Two such materials is (calcium sulfate) Gypsum and magnesium sulfate (Epson salt). There are many GH sulfate boosters on the market Seachem Equilebrium is one such product. Other GH boosters use calcium chloride and Magnesium chloride. Brackish water is a mix of fresh water and ocean salt water that is often founding river deltas Guppys prefer this water. So a chloride GH booster might be the best choice for your aquarium.
Note many GHbooster also contain potassium. Potassium doesn't affect the GH value of your water. Also potassium is also pressent in fertilizer. Other GH booster contain other additive such a calcium or sodium bicarbonate to increase KH and PH. To achieve a specific KH and or achieve a specific PH. My preference is to have a GH booster that has just calcium and magnesium, use a plant fertilizer to add potassium, and a sperate KH booster if you have a specific KH target in mind.
Other products have Calcium magnesium, and other ingredients to match water parameter the fish experiencesinits natural environment. The Seachem cichlid rift lake water conditioner contains Ca and Mg chloride as well as some sodium potassium, iron zinc and iodine All of which fish need for good growth and health.
