Fortunately despite the small size, there are still several food options for you to choose from. Some tiny babies especially those measuring only few millimeters; you can use microscopic organisms which you can’t even see yourself. First, what you need is a transparent container and some lettuce leaves, make sure you properly wash those and leave it under full blaze of sun so that the leaves will soften and decompose and that is where the infusorian microorganisms will grow. Don’t worry if you can’t see them because all you need is to use is a pipette or straw and then squirt the cloudy solution taken from the container of rotting lettuce and aim it close to the group of baby fish gathered together.
Another option which you can use is the cheap microworms. I would definitely recommend using these for those growing live bearer baby pet fish and for those small hatchlings like the baby goldfish, I would suggest you feed them infusorians for at least 2 weeks before you use microworms. I’ve written a comprehensive guide on how to get cheap and continuous supply of microworms just by regrowing the culture and making sure that they regenerate back the numbers. Apart from using microworms or infusorians, you can also mix in some dried fish pellets to add some variety to the diet by soaking the pellets in a small container until you can crush it until it turned powdery.
As your baby pet fish grow bigger, you should start to offer them different high protein variety so that you can continue to fill their stomach and make them grow faster. Generally one of the best foods is to grow brine shrimps (artemia) which are actually the eggs of saltwater shrimp which you will need to hatch and then feed it to the baby fish. Basically the brine shrimp culture need at least 2 days to hatch and in order to provide continuous supply of this food, you will need to prepare several containers to ensure there are sufficient amount to last for days. One of the benefits of using brine shrimps is that the shrimps will survive for few hours in water trying to escape and this will entice your baby fish to chase after them and make them active. This is basically the best food around and if you notice that your baby have a round stomach filled with brine shrimp their growth rate can be at least 1 cm in length achieved in a month.
Finally if you are staying close to areas with ponds and stream, that would be even better because you will have the chance to collect daphnia which thrive in green water. For further information, refer to choice of other live aquarium fish foods. You might also want to find out about different types of fish species.