What To Feed Baby Fish
Your fish have spawned and you now have baby fish, tiny little fish swimming about in the tank. Congratulations! If you planned this, you should already have some live foods for your baby fish ready. If you do not, the first line of DIY fish food for baby fish is hard boiled egg yolk mushed up in water. Be careful not to overfeed this and be aware that some baby fish may reject it as it is not 'live'. Baby fish are born with the instinct to hunt, and some of them are better than others at recognizing food that is not alive.
Infusoria are microscopic microorganisms. You can make an infusoria culture simply by putting some grass or other organic material in a jar of tank water. I have written a longer, more detailed guide, How to Make Infusoria here, and I advise you to read it and get a culture started. It is an excellent means to getting small, live food in the tank for your fish when they are too small to eat much else. Be aware that infusoria will only sustain your fry for a few days to a week at most, and that you will need to change to other live foods after that point.
Microworms are very easy to culture. If you are going to use microworms a lot (not reccomended because they have limited nutritional value after the first week or so of life,) make sure that you have a bare bottomed tank and / or snails. Snails will clean up leftover microworms and stop them from decaying in the bottom of the tank. Be aware, if you are spawning Betta fighting fish, you may end up with baby fish with no ventrals if you only feed microworms.
It is best to buy a little starter culture from someone and then you can easily grow your own microworms either on a bed of oatmeal and yeast or mashed potato and yeast (all you need do is make a little plain oatmeal or mashed potatoes, mix in a little yeast for extra food, spoon it flat into a small plastic container, then add the starter culture to it. Within a couple of days, you will see microworms climbing the sides of the container, where they can be harvested.
BBS (Baby Brine Shrimp)
Baby brine shrimp are the best food for most baby fish, though you have to make sure that you don't overfeed some sensitive species, like Bettas. Baby brine shrimp are very nutritious, relatively easy to culture and harvest (I recommend buying a shrimpery, if only because it makes life simpler and easier, and I also recommend having two, so that one batch is ready whilst the other is coming along. BBS take between 24 to 36 hours to hatch, so if one batch fails, you need to have a back up in the form of another batch, micro worms, or other large food. Here's my guide for making a BBS hatchery.
A powdered dry food for baby fish, this was recommended to me by an expert. Easier than culturing live foods, you can feed it so baby fish about the size of a newborn guppy. It will provide nutrients and minerals that may be missing in your other live foods and it is also very convenient.