Separating newly hatched fry from 2 week old fry

[Minifish]

Hi Guys,

So I have hatched some fry only issue is first batch only 1 survived which is almost 3 weeks old ! But daily I am getting newly hatched fry . As the one 3 week old is much larger and more developed than the hatchlings . I put the one in a breeder box in the fry tank as I was concerned it may eat the new hatchlings . As that one is too large to get out the holes. However the hatchlings keep getting in the breeder box . I remove them and they go back in. 
 

It’s a 10 gallon with some fake plastic plants and some duck weed with a few rocks . See photo . the largest fry is approx 6 mm and hatchlings are 1mm . Is it best to let the big one into the fry tank then the hatchlings can go in the breeder box to escape or will the big one eat them anyways .

many thanks 

[Guppysnail]

What type of fish fry?

[TOtrees]

I hope we're able to help 🙂

I'm having a hard time imagining that a 3 week old fry will be big enough to eat a newly hatched. Their mouth just isn't big enough. I don't know of any fish with that kind of growth rate. At least not in a captive aquarium environment. Maybe in a sun-powered micro-fauna rich environment, but not in a small indoor tank. 

Also, not sure about your measurements. Eggs might be 1mm, but once they hatch, even my tiny furcata rainbow and ricefish fry are 4-5mm,  what are you hatching that's only 1mm? 

Is your photos supposed to show some of the babies? Not seeing any can you point them out? 

PS the "duckweed" is salvinia 🙂