A method is described for improving water quality or correcting and adjusting important chemical parameters of the water in biological tank systems, such as aquaria (warm, cold, fresh or salt water), garden ponds, carp pools, water gardens and large aquaria (zoos, public aquaria) whereby the following are added to the tank system individually or arbitrarily combined: a) for reduction of phosphate concentration, at least one highly or poorly soluble Al<；3+>；, Fe<；3+>；, TiO<；2+>；, ZrO<；2+>； or Ca<；2+>； salt of an organic carboxylic acid, optionally mixed with an organic carboxylic acid； b) for reduction of nitrate concentration or limiting nitrate increase, at least one water soluble N-free biologically degradable organic compound； c) for an increase in carbonate hardness or the HCO3<；->； concentration, at least one alkali or earth alkali metal salt of an organic carboxylic acid； d) for an increase in total hardness or the concentration of Ca<；2+>； and Mg<；2+>； hydrogen carbonates, a mixture of at least one Ca<；2+>； and Mg<；2+>； salt of organic carboxylic acids and e) for an increase in CO2 concentration at least one biologically degradable compound and an appropriate single- or multi-component product.
