Research Reference:

Catalano, Devan N et al., 2019. The Effect of Goldfish (Carassius auratus) on Water Quality in Horse Stock Tanks. Journal of equine veterinary science, 79, pp.73–78.

Key "takeaways" :

Keep tanks clean. Water odor, temperature, turbidity and color will determine intake.

Horses should drink  1 gallon of water per 100lbs or for a 1,000 lbs horse = 10 gallons per day. Preferably 10 to 15 gallons/day. A horse eating fresh grass (pasture) will require less water while a horse eating hay usually requires more water.

A mentally and physically thriving horse is usually given required amounts of forage, water, movement (turnout) and social interaction with conspecifics.