To provide a water quality improving agent for improving water quality so as not to degrade freshness of sea life such as fresh fishes, especilly keeping color of the surface of the body of fishes close to that of natural sea life and further so that the quality of meat is inferior to natural fishery products.

The water quality improving agent contains various kinds of salts such as chlorides of sodium, potassium, magnesium and calcium which are the main components of seawater, contains rhodium as an effective component for improving physiological activity of sea life and contains palladiun or ruthenium or both of platinum group elements. Dissolved oxygen in seawater is raised by the prescribed platinum group elements and the physiological activity of the fishery life accepted in the body by branchial respiration to improve the quality of meat and body colors. The water quality improving agent is mixed with ice water in transporting water tanks or ship store houses of fishing places, so that freshness of fishes is maintained.