Can goldfish help keep horse’s water clean?

It may come as a surprise to see goldfish in a horse’s water trough. But in some places, it is a popular practice. Not only are the fish thought to slow the build up of algae, but it is claimed they also help control mosquitos by eating the larvae.

In fact, a recent survey of 672 owners or horse carers found that 44% had used fish to keep the water tanks clean at some time, and 18% currently used goldfish in their horses’ water.

Large water troughs / stock tanks tend to accumulate algae over time if not cleaned regularly. They can also become breeding ground for mosquitos.

But can goldfish really help maintain the quality of the water in horse water troughs? 

The survey formed part of a study by Devan N. Catalano and colleagues at the University of Minnesota.

Six adult horses were kept in a drylot which contained large (379L /100 US gallon) plastic and metal water tanks. Five fish were placed in one tank. After a month the tanks were cleaned, and the fish moved into the other tank. 

During the study, the research team monitored the water quality daily, recording total dissolved solids (TDS) and water turbidity (NTU). Once a week they measured the water chlorophyll-a content (the pigment responsible for the green colour of algae).

They found some differences in water quality between the two types of water tank. TDS was lower in the plastic tank, but the metal tank had lower turbidty and chlorophyll-a.

Tanks containing goldfish had lower total dissolved solids, but there was no other difference in water quality between tanks with or without fish.

Horses appeared to have no preference for either type of water container, or for the presence of absence of fish.

The researchers conclude that “goldfish do not improve water quality except for total dissolved solids.” They add: “frequent cleaning is important, especially in warm months and with plastic tanks.”

For more details, see:

The Effect of Goldfish (Carassius auratus) on Water Quality in Horse Stock Tanks.

Catalano DN, Heins BJ, Missaghi S, Hathaway MR, Martinson KL.

J Equine Vet Sci. (2019) 79:73-78.

doi:10.1016/j.jevs.2019.05.016.