So, the trick is to make sure the grounded flake powder of food is able to sink gently to the bottom, and let the fry know food is coming down from above, right?
Won't they avoid eating the food, as they might think the food is spoiled from above?
Nope. Just like humans can taste and smell when our food is rotten - fish can tell the difference between rotten food, and fresh food that just doesn't float.
I had new Molly Fry (same general fish type as guppies) about 1 month ago, and I fed them a combination of:
"Hikari First Bites" - looks like putting sand in your water, but they gobble it up.
Powdered flake food - just put it in a Ziploc bag and grind it up with your fingers.
Fresh Brine Shrimp - Made a hatchery from instructions on YouTube, bought the eggs, and
(then hatched the brine shrimp using the instructions before) adding them to the tank.
Baby Brine Shrimp are great for baby fish because:
--a. The shrimp swim around a little bit, which makes the babies chase them and get exercise.
--b. The shrimp are alive when you put them in your tank - so they don't go bad as fast as dead food.
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Ultimately, feeding your fry with enough food is going to be a bit messy until they're old enough to clean their plates. That's why people usually keep fry in breeder boxes, or in separate tanks, to keep from making their main tank messy.