Every Friday trimmings of hornwort and elodea go from the main tank into my window tank. Earlier this week I found a tiny baby fish just above the substrate! I'm pretty sure it's a serpae tetra that hatched from an egg stuck to those trimmings. This time I caught it and put it in a mesh breeder box hung inside the tank. Put all the hornwort and elodea floater stems in there, three stones with subwassertang on them, and loose subwassertang from a planter outside that was a tentative idea for a little container pond. (It's only been out there a week, I had just thrown some extra hornwort, subwasser and duckweed in it, thinking of getting some water lettuce and one very small fish... never did anything yet). But figured the plants from that planter might have more microscopic things on it for a baby fish to eat. I have seen it picking at the subwasser a bit.
I'm feeding it egg yolk, since of course I don't have infusoria or green water or newly hatched brine shrimp or any of that microscopic stuff you're supposed to have ready to feed very young fry right away. Got the idea from the forum. Put a bit of boiled yolk (smaller than a pea) into thin, old bandanna and twisted it into a little tight lump. Held with one of those little hair bands. Lowered that into the water and when the fry came near, squeezed it with the tip of planting tweezers. A cloud of yellow came thru into the water I could see the fry biting at bits too small for my eyes. I saw its belly get nice and plump. Then it seemed to loose interest so I gently siphoned out the remainder in a straw and gave it to the adult serpae tetras in the main tank. I kind of thought those bigger fish wouldn't be interested in such tiny bites, but they swarmed all over dashing after those yellow bits. I don't think I'll do that again, though. Egg yolk is too fatty for adult fish.
From more reading, I have to feed it three or four times a day for the first two weeks, or it will starve. Must have high-protein food (so not sure if green water from mashed pea would be ok, probably not enough nutrients). Really the best thing is infusoria- I've tried to start some. Put tank water in a jar with a few drops of plant food and dead leaves from my main tank, out in the sun. Also in the house I stated four jars with tank water and each has a different food for the culture: banana peel, crushed lettuce leaf, grass, bit of potato. With screen over it to keep out bugs. Maybe I can keep the fry going on egg yolk for a week and then I might have some actual infusoria for it. After that it can transition to microworms, after another week or two of that, can start eating powdered prepared foods (dust from the bottom of my flake and pellet cans). I found someone who can give me microworms day after tomorrow (guy from the new local fish club), so maybe I can have a supply of that ready when the fry is big enough to eat it...
I've finally figured out how to recognize mosquito larvae in a puddle that forms on a tarp under my deck- I collected some today and fed them to my adult serpaes- they went nuts for it. Some of those larvae were so small, like tiny clear threads. Wonder if I could feed those to the fry when it is a bit bigger? or some newly-hatched red wigglers from my worm bin? I hope between all those things I have enough to keep it alive until I can release it back in the tank, or feed it powdered commercial foods which is easier. I don't mind the little challenge. At least, so far.
I'm told on the forum that trying to feed it in the mesh container will just pollute the area it's in- any tiny drop of food I put in will be too much for one fry to consume- I'd hoped that siphoning out the bottom of the container, adding a snail or two to eat leftovers and doing small partial wc on the tank daily would keep the environment clean enough, but maybe not. One of my cories swims upside down mouthing the underside of the mesh box when I feed the fry- I wonder if he's actually getting any food thru there.
If I didn't have Fabio in here I'd put mesh or sponge over the filter intake and just let it grow up in the tank. Already this feels like a lot of work for one tiny fish (my kids are super excited about it though).