Total Hardness (GH) The total hardness shows the calcium and magnesium salts dissolved in the water. No recommendation can be given for an optimal level. Most species prefer medium hard water (8-12° dGH). East African ornamental fish, however, live in hard water (15-30° dGH). To reduce the hardness we recommend aquavital black peat or aquavital granu-peat. To increase the water hardness use calcium contained in stone or gravel (e.g. coral sand or marble stone).