I kept my fry in the breeder until they started showing color. They're about 4 months old now and the biggest ones are about 2.5-3".
I gave up and let them decorate the tank for me.
It's been awhile since I used a breeder box. Too much work.
The first batch about 3 months, were moved from the 20L to a 33L at about 2 months. The 2nd batch about 2 weeks now just a bit smaller than yours are in the 20L.
I would move them, even a 10g would be better for them and less work as mentioned.
Learning all the Time.
Aha! You have a pair that spawns quickly too. As mentioned, "like Rabbits" if I leave them together.
I target feed, 3-4x a day using a pipette, I am sure I have overfed, but that is one of the reasons I do 2-3 water changes per week. I carefully use a small siphon tube, into a bucket to ensure I do not lose any. I do find them on occasion.
Right now, I have probably 4-6 dozen about a week younger than yours. They are doing fine, also they are the only fish in the 20L. Never had Amanos with them, so no help there.
But if your fry are anything like mine they are inquisitive and will test anything to see if its food, they have no fear. I even noticed one today going after a FDBW had it in it's mouth, the worm was bigger than it was. The parents are "Great" protectors and teachers but you know how kids are. LOL
In fact, I did a water change today and changed out their sponge filter and added a piece of driftwood from another tank. The fry swarmed it, obviously there are some micro-organisms on it they like..
Just made a deal with the LFS for the 3-4 month batch, dropping them off on Weds.
What dose did you use? I mixed 1 gram with 500ml water and then dosed 50ml per 10g (100ml for the 20L) since I do not have a gram scale.My experience with pancur for planaria is it did no damage to the snails as it was only a 2-3 day treatment at a very low dose ..
Good luck !