If you saw several fish around a single larger fish, they were in the act of spawning. It's commonly known as a "rock fight". Several males will gather around a female toward the surface and agitate her. That stimulates her to release her eggs into the current. You should then see the water have white clouds around the smaller males which is the sperm. The eggs are fertilized and tumble in the current. They will hatch in about two days if they tumble at the right speed.