Tropical fish diets in the wild typically include tiny invertebrates, worms, and algae. I like to start new tanks by adding some leaf litter from my yard. The aquarium light should be in enough to encourage some algae growth, and rotifers, copepods, daphnia, etc will quickly hatch from omnipresent microscopic eggs on those leaves. Adding a tiny bit of yeast powder (like for bread) will cause these organisms to quickly multiply, and after a couple weeks, they're everywhere. They keep the water super clear by eating excess algae and biofilms, helping keep sensitive shrimp healthy, and they serve as a constant food and activity source for fish assuming the tank isn't overstocked and has hiding spaces for the microfauna to safely reproduce. Many microfauna species are large enough to observe with the naked eye, so you'll know when they're thriving.