I can not stress enough how important it is to have a separate cage for 'quarantine' when you bring home new birds!
A bird may not show any obvious signs of being sick but doesn't mean it is 100% healthy. There are sicknesses that do not show any signs until it is too late and have most likely spread to everyone.
You want to check the feet for cuts or lumps. If there are any open cuts or a lump with a scab (bumble foot which you can see how to treat in another post 'How to Treat Bumble Foot') you want to clean the feet. You can go to any grocery store and get Epsom Salt and put some in luke warm water then just let the bird soak for a bit. I like to let my ducks sit in the water until the water goes cold but that's just how I set the time. Then once you decide the bird has soaked enough you want to treat a cut with blu kote which can be found at any livestock supply store such as Tractor Supply. You also want to get some gauze and place it over the cut then wrap the gauze to the foot with some self adhering wrap. I like to use the wrap for horse legs that you can find at Tractor Supply and they tend to have some of the colors that didn't sell on sale or even a '3 for $5' deal. You can use that self adhering wrap for just about any injury your duck gets, I have used it from a broken leg to bumble foot. The next thing you want to do is to put your duck in a clean cage where you can keep an eye on him or her. Then you can allow your bird to soak once a day and replace the wrap with clean supplies.
If your duck is showing other signs of illness such as a runny nose, watery eyes, or lethargic I suggest using electrolytes in the water and some probiotics. A runny nose can be as little as something be in there such as dirt or as major as a respiratory illness. Watery eyes tend to be an eye infection or dirt, you can fix most of these with giving then a deep dish of clean water to cleanse their eyes and noses but what is this white foam in my ducks eye?? Well that also tends to be some stuck dirt or it can be a blocked tear duct in which some times a duck can dislodge what ever is in there but can sometimes require a vet going in and cleaning out the duct. With a lethargic duck you want to give electrolytes or nurti-drench with lots of 'treats' such as strawberries, tomatoes, cantaloupe or peas. Most times that can perk up your duck.
If your duck has little bugs on it's skin it can either be fleas or mites. If fleas you can treat with flea drops for dogs... BUT only apply a SMALL amount under each wing. If mites you can give your birds a diatomaceous earth (DE food grade) bath by sprinkling a little bit on them or even rubbing it on to the bird. Be warned it dries out skin and can be a breathing hazard if it gets too dusty! The next step I would do is remove any and all bedding, clean the pens, place DE and new bedding for everyone.
I quarantine any bird for about 30 days before allowing them in with everyone else.
I believe that is the basics. If your bird looks really ill and nothing is helping consult a vet asap!