One remaining duckling is still alive and tonight it seemed less sickly and I think it doesn't limp as it used to. It still rather swims and rests on the ground and is not nearly as energetic as the large duckling, but it does show some signs of improvement.
I did a few changes in their diet. Since I can't prevent them from eating slugs, I make sure they eat more feed and I started giving them a lot more greens. They don't like peas or most of the green plants, except for chick weed and fat hen which they love. I give them a few slugs and I bring them next to the pond. The large duck instantly jumps into the water and eats them inside. The small one usually drops it into the water this way and since it sinks to the bottom, I fear the water will become filled with dead slugs eventually. They don't eat them when they start floating in a day or two.. I lure the small one out and give it the smaller slugs that are easier to swallow or offer them chopped slugs on a plate while they are in the water. This way, if the small one drops a piece into the water, the large and always hungry duckling grabs it. She even started swimming under water. Feathery beast! This way, none had issues and I've noticed they both refuse to eat a slug if there's no water around. LArge duck takes it and walks into the pond, the small black one just nibbles a few times and leaves it there.
They refuse to eat fresh peas. To prevent them from becoming niacin deficient, I tried adding yeast on top of their feed. They hate that as well. For now, I'm hoping they will get enough niacin from greens and slugs (they don't roam around as much now so they eat less insects).
We've had some serious thunderstorms last night. They went to their coop on their own, but they rushed out as soon as they saw me walking by. After that, they stayed under dense bamboo during the rain and didn't seem to be bothered by it. Some time later, I managed to make them go inside and I closed the doors, so they remained inside for the night. Tonight, I have found them in their coop at dusk and I only needed to close the doors for them.
Here's a video of their cleaning job after their evening meals and before their sleep.
The small one is also getting its feathers.