The general hardness gh of water has a direct influence on the regulatory functions of fish fish skin gills and eggs plants and microorganismsr a favorable gh value for most ornamental fish between 6 and 16 dh degrees of general hardnessr often forgotten or lost in significance to testing ph ammonia and nitrite general hardness is vital to selecting fish suitable to local water supplies or testing the performance of advanced filtration methods including reverse osmosis or deionizat