Hi, Jae, Welcome to BackYard Chickens!
Are the ducklings always being housed on the floor shown in the video? It's quite unlikely that you're going to see niacin deficiencies in a bird this young, if they are being housed on that floor I may be more inclined to think that they may be suffering from splayed legs, which is mostly due to a slipper bedding material. First, get them on some towels. As far as the gasping, we'll need some more information, but generally, with waterfowl, panting is usually caused by too much heat, but since you say they are dropping one by one, it sounds like we may be dealing with something more serious here.
Could you post a picture of your setup, what's the temperature, and a picture of their droppings?
Are the ducklings always being housed on the floor shown in the video? It's quite unlikely that you're going to see niacin deficiencies in a bird this young, if they are being housed on that floor I may be more inclined to think that they may be suffering from splayed legs, which is mostly due to a slipper bedding material. First, get them on some towels. As far as the gasping, we'll need some more information, but generally, with waterfowl, panting is usually caused by too much heat, but since you say they are dropping one by one, it sounds like we may be dealing with something more serious here.
Could you post a picture of your setup, what's the temperature, and a picture of their droppings?
We're so happy you've decided to join us 
