Goldfish: Do They Help Keep Your Horse’s Water Tank Clean?

Have you ever looked into a water trough for horses only to be surprised by the presence of goldfish? Some horse owners swear by goldfish, praising them for their algae-eating ways. Others eschew goldfish as management tools, choosing instead to keep waterers tidy through regular scrubbing.

To evaluate the efficiency of goldfish in maintaining water quality in tanks, researchers measured total dissolved solids (TDS), water contaminants, and algae growth in large (379-liter) stock tanks, either metal or plastic, with and without goldfish from June through October. Stocking rate of goldfish was based on the recommendation of 76-95 liter of waters per goldfish; hence, researchers kept five goldfish, all 3-4 cm long, in each tank at all times.

Total dissolved solids is a measure of any substance in water, organic or inorganic, other than pure water molecules. In general, TDS values under 1,000 mg/l are safe; values between 1,000 and 2,999 mg/l are typically safe but may cause diarrhea in unaccustomed horses; values greater than 3,000 mg/l may hinder water consumption; and values in excess of 7,000 mg/l indicate that such water should not be offered to horses. Total dissolved solids were measured daily using a handheld meter at nine distinct spots and water levels in every tank. The degree by which water lost its transparency due to particulates, called turbidity, was measured weekly. Water samples were also taken weekly to test for chlorophyll a, an indicator of algae. (More details of how the study was designed can be found in the study abstract.)

What did the researchers find?

• Goldfish did not improve measures of water quality except for total dissolved solids.
• In comparing metal and plastic tanks, metal tanks had lower turbidity and chlorophyll a.
• Total dissolved solids and turbidity increased as water temperature rose.
• Horses did not show a preference for water with or without goldfish.
• A high mortality rate of goldfish throughout the study.

Researchers could not “conclusively recommend using goldfish as a management method for maintaining water quality in water tanks,” though the study did underscore the importance of frequent cleaning of troughs, especially when using plastic tanks in warm seasons.

Catalano, D.N., B.J. Heins, S. Missaghi, M.R. Hathaway, and K.L. Martinson. 2019. The effect of goldfish (Carassius auratus) on water quality in horse stock tanks. Journal of Equine Veterinary Science 79:73-78.