My horse doesn't seem to drink much water. I notice him going down to the water every now and then, but it seems to take forever to empty. It's a 70 gallon tank, but since it's on a slight hill, full, it's probably only about 60 gallons. It's been over a week since I filled it, and it's not empty yet, and a lot of what's gone is probably evaporated, since it has a water heater in it.
How much should he be drinking? Is it bad if he's not drinking enough, or should I just trust him to drink when he needs to?