In case anyone comes up against this, It was botulism and it came on very fast. The ducks are surprisingly alive thanks to this woman's text from another forum (I used the Epsom salt option with great results):
"My suggestion at this point, as it does sound suspicious for botulism, is to do the following for her (if you are able). Give her either a charcoal flush (2 rounded tbsp crushed into a fine powder, mixed with 1/2 cup water), an epsom salt (magnesium sulfate) flush (1.5 tsp epsom salts in 1/2 cup warm water) or a molasses flush (1 tsp molasses in 1/2 cup water). You can even do a charcoal/ep salts combo.
Get a length of tubing (difficult to do if the stores are closed...) from your pet store - air hose tubing from the aquarium supply store is your best bet. measure from the end of the bill to the base of the neck, then cut the tube 6 inches longer than this length. Mark on the tube the distance from the tip of the bill to the base of the neck (crop) so you know when you're in far enough. You'll also need a luer-slip tip syringe - this is the type of syringe with a tip that a needle slides onto, as opposed to the type where the needle twists onto the end and locks into place. get a 60 cc syringe if possible. Now, fill the syringe full of fluid and attach the tubing to the end. Open the bird's bill and look inside: at the back of the tongue (you may need a flashlight to see this) you'll see a hole that is the opening to the airways. This is called the glottis. You'll want to slide the tube down the throat but DO NOT GET THE TUBE INTO THIS HOLE as any fluid in the bird's airways will be fatal. It's quite easy to get the tube in- - just wet it and slide it down, avoiding the glottis. If you run into resistance, back the tube up and reposition the neck. Try again. Once the tube is in far enough (the mark on the tube is at the tip of the bill) then slowly give all the fluid. If you need to refill the syringe, do not remove the tube, but just the syringe, and refill it quickly. It helps to have someone holding the bird while you're doing all this. Give a full sized duck about 60-120 cc at a time, watching to make sure that no fluid is coming up the throat. Repeat the flush in about 3-5 hours (when the crop is able to take more fluid) and every 3-5 hours after that for the first 24 hours. Keep a bird with botulism cool, not warm, as it slows the metabolism of the toxins (not cold... just not under a heat lamp). Be careful that you don't overfill them with fluid as they often have a hard time swallowing due to paralyzed muscles in the esophagus. If you do not have the ability or materials to tube feed the duck, try syringing in as much charcoal/epsom salt/molasses flush as you can every hour.
There are a couple of diseases that can cause similar symptoms, as well, but if you are suspicious of botulism, the faster you can treat it, the more likely your bird will survive. If it's another disease, you have a bit more of a window to treat them, so best to be on the safe side and treat for botulism first when you're not sure.
This is the other possibility that I'm wondering about.... Have a look and see what you think.
Let us know how your duck fares the night.... good luck...