Cleaning your horse's water tank

Total dissolved solids (TDS) is the main measurement used to assess water quality for livestock. TDS values below 1,000 parts per million (ppm) are considered safe for all classes of horses. 

Water turbidity, or the overall clarity of the water, is another way to assess water quality. Typically, water should have a turbidity value less than or equal to 5 NTUs (nephelometric turbidity units) for human drinking water.

There are no ranges or limits that exist for horses or other livestock. However, a fairly clear water source with a turbidity value of 25 NTU is considered acceptable for a horse. 

Plastic or metal tank?

During a 2019 University of Minnesota study on whether adding goldfish to water tanks improves water quality, we found that using metal tanks may promote lower turbidity and algae than plastic tanks, meaning water quality may stay higher longer in metal than plastic. This could be due to rough plastic surfaces being harder to clean.

Water temperature affects water quality 

There was no difference in water temperature between the plastic or metal tanks in the study. For every 1 degree increase in air temperature, water temperature increased by 0.60 degrees.

For every 1 degree increase in water temperatures:

• TDS increased by 0.80 mg/L.

• Turbidity increased by 0.65 units.

Our study, published in the Journal of Equine Veterinary Science, could not conclusively recommend using goldfish as a management method for maintaining water quality in water tanks.

Study highlights:

• Goldfish do not improve water quality but may reduce total dissolved solids.
• Metal tanks had lower turbidity and chlorophyll a than plastic tanks.
• Total dissolved solids and turbidity increased as water temperature increased.
• Frequent cleaning is important, especially in warm months and with plastic tanks.

Testing water quality control when using goldfish

We conducted an experiment to test the effects of two tank types (plastic and metal) and goldfish on water quality. 

• There were four 100-gallon stock tanks, plastic and metal, both with and without goldfish. 

• We used a stocking rate of 20 gallons per goldfish for a total of 5 goldfish per tank. 

• Every day we recorded air temperature, water temperature, TDS, and water turbidity.
• We sampled water to determine algae once a week. 

• After 28 days, we set aside the goldfish in holding buckets and scrubbed the tanks clean by hand and refilled them. We acclimated the goldfish to the temperature of the fresh water in their holding buckets and then released them into the water. 

• We added water to all the stock tanks when any of the tanks were halfway gone. We used this value to determine if horses preferred one tank over another.

Did goldfish make a difference? 

• TDS gradually increased throughout each 4-week period, and plastic tanks with fish had lower TDS compared to metal tanks without fish. 

• Goldfish did not affect water turbidity. However, metal tanks had an overall lower turbidity value compared to plastic tanks. 

• Goldfish did not affect algae content of the tanks. However, metal tanks had less algae compared to plastic tanks. 

• There was no effect of tank type or goldfish on horse water intake. Horses drank out of all tanks equally.

• There was a high death rate among the goldfish in the study.

*Adding goldfish to the tank had no effect on water turbidity or chlorophyll a (chlorophyll a indicates the presence of blue-green algae).

We conducted a survey of horse owners and managers available from October 31 to December 2018. 

The survey consisted of five to nine questions regarding water tank and fish use, along with five demographic questions. The first survey question asked if a participant had previously used, currently used, or had never used goldfish or other aquatic species in tanks. Based on the response to the first question, participants were asked slightly different questions. However, all participants were asked the same demographic questions. 

Survey results

We received a total of 672 completed surveys.

When asked about prior use of goldfish in water tanks: 

• 56 percent of respondents had never heard of nor used fish species in their horses’ water tanks. 

• 26 percent of respondents had used fish before.

• 18 percent of respondents currently used fish.

Of respondents that used fish, 88 percent used common goldfish with a stocking rate of 1 to 6 fish per 100-gallon tanks. Of those who no longer use fish:

• 22 percent stopped because they felt fish did not work or did not reduce time spent cleaning.

• 43 percent stopped because the fish died.

When asked about cleaning water tanks:

• 27 percent of horse owners and managers indicated they cleaned water tanks once per week.

• 12 percent had never cleaned their horses’ tanks. Of individuals who had never cleaned their tank, 86 percent currently used, or had previously used, fish to maintain water quality.