Jim Wendler (5/3/1 creator and author) often recommends ZMA ("ZMA is a natural mineral supplement made up of zinc, magnesium aspartate, and vitamin B6. Zinc supports your immune system and muscles. Magnesium plays a role in metabolism and muscle health and helps manage sleep. B6 may boost energy."
https://www.webmd.com/vitamins-and-supplements/zma-supplements) for recovery purposes. It will supposedly improve quality of sleep and he said : "This is one of my favorite supplements for the simple reason that it allows me to sleep longer and without waking up. Furthermore, I swear it helps me have larger amounts of ejaculate. And you can't put a price tag on that."
I just started taking it, and I don't know if I'm that impressionable, but I too have noticed the slight increase in volume.
Hydration is definitely a factor as many have stressed on it