That article is so, so sad. I hope they find the owners of the horses and prosecute to full extent of the law.
I have 7 horses (my 6 and Amarea's horse) drinking out of a 150g stocktank. I have used goldfish in the past, but don't anymore. With this many horses drinking out of the tank, they can drink it more than 3/4 down by mid-afternoon most hot days. I dump the tank, spray it out, and refill. I also add about 4 cups ACV to the water every dump/refill, keeps away the mosquito larvae and also helps somewhat with inside-out fly control with the horses, as well as encouraging them to drink more.