If mites they will look like tiny pepper and they move fast. never had any on my water fowl just chickens a few times, but if they are swimming daily I doubt mites. water helps keep the parasites off water fowl.
When you put them out at the pond be sure to gradually take them off of feed though so they aren't starved being they will be use to you feeding them it will take time for them to learn to forage for themselves. you could try some poultry dust just be careful using it so you nor them inhale it. do their bedding too.
