Many thanks for the good advice!
Although I hadn't considered nutrient deficiency, what you write makes sense, and I see how that can explain the symptoms I've seen.
I will follow your advice, feed them (live) artemia, and give them more varied food.
I also didn't realise corydoras were...
Thanks for your feedback.
attempt 1: a combination of flakes and tetra tabimin (tablets), daily
attempt 2: tetra tabimin tablets, one tablet once or twice daily. presumably they were able to scavenge food f rom between the plants as well
attempt 3: tetra tabimin tablet, 2 times...
So I've tried raising corydoras fry 3 times now, all three times significant death of the fry after ca. 8 weeks.
ca. 40L tank, cycled and established with heater and filter, planted
ca. 30 corydora fry and 6 cloud minnow fry (all about the same age)
20% water change...