Goldfish More Hassle Than Help in Horse Water Troughs?

Anecdotal evidence suggests placing common goldfish in horses’ water tanks will help keep the troughs clean and free of algae. But Devan Catalano, a PhD in animal science at the University of Minnesota, in Minneapolis, recently determined that’s not necessarily the case, also finding that certain water troughs are better suited for staying clean than others.

Catalano presented her study results at the 2019 Equine Science Society Symposium, held June 3-6 in Asheville, North Carolina.

“Algae is a growing problem” during warm months, she said. “Small quantities can turn water green and affect horses’ intake, while large quantities can be toxic. It’s hard to control, and cleaning is time- and labor-intensive.”

The theory behind placing live goldfish in tanks is that they’ll handle some of the cleaning by eating the algae.

“We horse people are resourceful, and if we can find a way to save time, we will use it,” Catalano said.

Her study goals were twofold: To evaluate whether goldfish could maintain water quality and to determine how frequently horse people use this method.

For five months (May through October) she kept plastic and metal tanks with (treatment) and without (control) goldfish in a drylot housing six horses. She placed five goldfish in each 100-gallon treatment tank—a number she came to based on goldfish stocking requirements.

Goldfish More Hassle than Help in Horse Water Troughs?

Each day Catalano measured air temperature, water temperature, turbidity (clarity), and total dissolved solids (TDS, a standard gauge for water quality; 1,000 ppm or less is considered safe for horses). Each week she measured chlorophyll A levels to determine algae growth. She refilled tanks when they dropped below half full and used that as an indication of horses’ water intake and preference. Every 28 days, Catalano cleaned and rotated the tanks to account for horses’ potential tank location preferences. At the end of the study period she found that:

  • For every 1 degree Celsius (1.8 degrees Fahrenheit) increase in air temperature, the water warmed 1.6 degrees Celsius (2.88 degrees Fahrenheit);
  • Plastic tanks with goldfish had the lowest TDS;
  • Metal tanks without goldfish had the highest TDS but were still well under the 1,000 ppm threshold;
  • Goldfish had no effect on water turbidity or chlorophyll A;
  • Plastic tanks had greater turbidity than metal tanks;
  • Plastic tanks had higher chlorophyll A levels than metal tanks; and
  • Horses showed no preference for tank types or goldfish presence.

So while goldfish had a positive effect on TDS, they did not affect water clarity or algae levels. Both tank types are suitable for horses, but Catalano said she would recommend metal over plastic based on her results.

She also distributed a survey on goldfish use in water tanks to horse owners. Of the 672 responses, she learned that:

  • 26% had used fish in their horses’ water tanks in the past;
  • 18% still use fish;
  • Of those who had used fish previously, 52% believed they were effective;
  • Of those currently using fish, 75% believed they were effective; and
  • 62% of respondents said they’d be influenced by research on the matter.

“While the survey indicates fish are a popular management strategy,” said Catalano, “I wouldn’t recommend it due to fish mortality (she saw a high mortality rate in the goldfish in her study) and the minimal effect on water quality.”