You can use goldfish to keep horse troughs clean!
Aquarists and fish owners seem to have trouble keeping goldfish alive in tanks. This is because they're relatively large fish for the size tanks that they are usually held in.
One interesting way that you can learn this is the fact that goldfish can thrive in cattle troughs. This seems a weird place to keep goldfish, but actually they help farmers keep the troughs clean from algae and other bacteria.
The reason why they thrive is because these massive tanks hold between 150 and 300 gallons of water, while most people keep them in tanks that are about 30 or 55 gallons. This deteriorates the water quality that they live in and can lead to their death.