HELP! something is wrong with my ducklings.

[threeducks3]

I bought two new ducklings about a month ago and two other ones a week later. I'm new to duck so I'm not 100% sure what type they are. This first two i know are domestic ducks (One girl and one boy) and the other two I think are mallards. About a week days ago my girl domestic duck started limping and we didn't know why(so we separated her ). Their was no sign of injury on either leg so we assumed it was a sprain, then a day later she couldn't stand up, and the third day she started breathing heavy with her mouth open and very fast and her sclera (the white part that surrounds the eye) turned pink and she looked like she was dying. She passed away that night.I have no clue what was wrong with her. She was eating fine and drinking fine up till the third day (when she passed away). I'm not sure if she had a birth defect or if she was sick but we always kept her and the others eating area clean and we would let them swim in the bathtub. We also took them out walking a couple of times and gave them grass to eat. She was never as active as the rest when we first bought her but she was eating and drinking fine and she would follow us when we went walking in the park. But she didn't squeak as much as the rest of them and she was very quite. When she was dying she flipped on her back and started kicking her legs in the air tell she stopped breathing (i forgot that the other ducklings were in the room because they got so quite and i think they may have watched her die).i've heard that the other ducklings will bite the tale of a sick duckling to make them leave so that the flock will survive. I'm a bit confused on that because they never bit her tale feathers. But the two mallards would always try eating the boy domestic duck's tale feathers (the news ones that were coming out). i assumed that because he was a week older that they were just biting him because his new feathers may have looked like food or something but now im not so sure. I do know that one of the mallards is a boy because he would always try fighting with the male domestic duck.But after they watched the girl pass away they all stopped fighting (except for a couple of times when the mallards would try to pull the new feathers out the domestic ducks tail). But i'm scared for them because the male domestic duck seems very fat and he was breathing heavy (but not like how the girl was) but the white around his eyes did turn pink. He is still eating and drink and we bought him and the rest of them some antibiotics and some duckling food. They all eat fine now except the male mallard who keeps eating poop and drinking water. He refuses to eat the new food so i gave him some oats, eggs, and a little bit of bugs to eat. But i'm scared for him too because i don't know if they are getting sick or if they are deppressed. and sometimes they poop different shades of green (with white) and brown (all different textures)..Is that normal? Is there something wrong with my domestic duck or my mallard? Can ducklings get deppressed? How can I get my mallard to eat the new food and stop eating poop? What was wrong with the girl domestic duck? was she sick or did she just have a birth defect?