While the second batch of fry succumbed to parental cannibalism, the older ones are doing well. My brine shrimp yields have been pretty good, so they have been eating fairly well. What's remarkable is the differences in growth rate among them. There are several fry that are considerably smaller than the average, and there are a couple that are considerably larger than average - most notably, one giant who is 2-3 times the size of its siblings.
I ran the filter for a little while again yesterday - although the fry were still using the entire water column, I was a little concerned about oxygenation and BOD (biological oxygen demand). The fry are unlikely to be air breathers yet, and there's sure to be other decomposable material in the water. Added oxygen is also going to benefit the bacteria involved in nitrogen cycling (which are mostly aerobes). Still, I don't like the leave it running too long, since the fry aren't yet strong enough swimmers to deal with the water currents set up by the filter.
So while still water is likely to reduce the energy demands of the fry, it also reduces the oxygen content of the water (and possibly the ammonia and nitrite concentrations). I don't know if it's a net benefit or cost. On the other hand, the Ludwigia seem happy with the stillness and the falling water levels. Several stems have emerged above the water level, and at least one of them has flowered. The breeding aquarium has taken on something of a pond appearance, which I really like.