Yeah, I've seen that in action. On my way out one morning, saw a dead guppy at the bottom. I had to get going, so I was going to get it out when I came home from work. Came home, looked, and no dead fish.
This 10 gal tank has only guppies, about 10 adults, and 20-30 half grown and younger fry.
And if I want to get the tank partly cleaned, as in most little bits of algae or what not, I don't feed them for 3 feedings, then the fourth is a smaller feeding. And I feed morning and night. Then I do the gravel vacuum. Works pretty good. I only do partial cleaning by fish once a month. 20-25% water change weekly, with a gravel vacuum.