Corydoras Fry at 1 month Starting to get ill

Hi Guys
Need some advice Please.
Ive been around here a hell of a long time now & kept Tropical fish for as long as i can remember.

Recently i decided to save some of my Bronze Corydoras eggs & see if i could hatch them. With success they hatched & are now 1 Month/5 weeks old. There were 25 born but only 15 survived to this day.
They are growing & eating well, with a diet of Tetramin flake, Mini bloodworm,Tetra Shrimp wafers, BBS & Fluval bug bites.
They currently live in a 10Gallon by themselves with a sand substrate, Heated to 25oC, Sponge filter & small airstone, P.H 7, Ammonia 0ppm, Nitrite 0ppm Nitrate 0ppm (source API Liquid test) the tank has 25% daily water changes this is an established tank.

In the last week i have noticed 1 by 1 my Cory fry were flicking on the sand & seem to be bothered by something, A day or 2 later they are floating in & out of consciousness & die overnight.
I'm unsure if it may be Gill related but the gills appear a little red. I don't want to treat for flukes incase it's not that

Will post pics below, But unsure if they help

Thanks for reading

Definitely not my area of expertise, but your cories look a little skinny. Perhaps they are not getting enough food (most of the time this is not an issue). The barbels look good, but the head looks like it is waiting for the body to fill out. Hopefully someone else will pipe in.