A composition for long term improvement of water quality in biological tank systems is described as: 1) at least one highly or poorly soluble Al³⁺, Fe³⁺, TiO²⁺ or ZrO²⁺ salt of an organic carboxylic acid, optionally mixed with an organic carboxylic acid; 2) at least one water soluble N free biologically degradable organic compound; 3) at least one soluble alkali or alkaline earth metal salt of an organic carboxylic acid; and 4) at least one Mg²⁺ salt of an organic carboxylic acid, optionally mixed with at least one Ca²⁺ of an organic carboxylic acid; and also 5) trace elements and vitamins, particularly water soluble vitamins of the B series. Changes in water quality determining parameters can be reduced, minimized or eliminated by using the described agent and thus a significant reduction in partial water change frequency or a clear extension of periods without water changing can be achieved.