Turns out my baby fish is older than I'd thought- it must have been a week already on tuesday. So I don't need to feed it three, four times a day and it doesn't need infusoria or egg yolk, it's already big enough to take pulverized food. I was pretty glad to ditch the egg and dump the jars of infusoria I'd tried to start (only the potato one was starting to go cloudy with life). I'm feeding it two or three times a day now, I see it picking at microorganisms pretty regularly and as long as the tummy stays round I think it's okay. Even in just a few days it is getting easier to see.
I upended all my fish food containers onto a sheet of paper (separately) and dumped all the powder residue into an empty container (poor quality flake I had thrown out a while ago but kept the can). Mixed all the powders together, I figure then the fry will get variety with each feeding. I'm told to feed it very sparingly as one fry won't eat much, so I cut the end off a toothpick to make it blunt, get it damp and dip just the tip in the food powder. Then into the water. It seems to be enough, yet even that little bit attracts the grown fish- yesterday two cories and Fabio were mouthing the bottom of the mesh box.
Snails are attracted too, they climb the sides of the fry box. (Maybe they are after the decaying hornwort needles that get trapped between the box edge and tank rim- I gently moved the box the other day to clean that area out). I have put a few more snails inside to help with cleanup- now I siphon out their poo more than food residue, but I think that's less toxic for the fry. (The baby fish is dwarfed by an adult trumpet snail- it's less than half the size!)
When I feed the fry in evening the room is too dim to actually see it. I put a little flashlight on the tank lid over the mesh box, so I can see if it is eating. Seems to attract it to the food area too.
I'm still doing small daily water changes, for the sake of my baby fish, as the nitrates are higher than I'd like- not sure why. Hornwort is struggling to adjust, dropping tons of lower needles and altering its top growth. Looks terrible- but I know it will recover or be replaced easily with more trimmings. None of the other plants visibly affected- I bet it's because the hornwort is such a fast grower, an 'indicator plant'. I think the regular wcs have depleted nutrients for it, but then its dying foliage pollutes the tank.