The most important factor in survival of baby koi fish is how the spawning process is accomplished. Koi are notorious for feeding on their young, so moving breeding fish into a separate aquarium for spawning is critical. Baby koi born in the koi pond are fish bait, literally. Moving the adult fish out of the spawning tank quickly after spawning is complete and before the eggs hatch is also crucial. Eggs will hatch within four days after being laid, so the tank needs to be monitored constantly and adult fish moved --- back to the pond or into another tank to recuperate --- when eggs are first discovered. Koi are very hardy fish and are easy to care for after they've hatched, if these precautions are taken to keep them from predator fish.
Provide spawning mops that fry will attach to.
Newborn koi fry instinctively try to hide and will attach themselves by the head to spawning mops in the tank. At this stage, baby koi have no mouth, swim bladder, fins or vent. Don't feed koi fry at this stage; they survive by feeding off their egg sac, which is still attached.
Use air stones to provide plenty of oxygen to the tank.
For two or three days of life, baby koi breathe by absorbing oxygen through the fine blood capillaries that surround the egg sac which is still attached to the gut. It's essential that fry receive enough oxygen. Monitor that air stones are functioning properly at all times or the koi babies might die.
Watch for fry to begin swimming.
After two or three days, fry will develop fins, a mouth and other organs. At this stage, they will head to the top of the tank, gulp a few breaths of air into their swim bladder. This allows them to begin swimming freely in the tank. Fry then gather around air stones, a sign they are ready to be fed.
Feed liquid fry food frequently for the first week.
Fry should be fed small amounts four or five times a day as they are developing. Also offer strained hard-boiled egg yolk for the first couple days to help increase the size of their stomachs.
Offer newly hatched brine shrimp larvae after one week.
The brine shrimp larvae are small enough for fry to eat at this stage.
Switch from liquid to powdered fry food at week two.
Continue to increase the size of food offered as fish grow for the next three or four months.
The main consideration is that the food is small enough for the fry to eat, has high protein content and is offered several times a day. Debris and waste need to be removed frequently, as fish develop the ability to eat.