The best de icer depends on where you live and how cold it gets.
We use 44watt deicers from amazon that are also good for bird baths, so it fits under the metal and 3 gallon watering containers. Because the chickens are in the barn in the Minnesota winters, this works good enough.
If the birds were outside, or in an exposed are, go for 125+ watts, also got ours on amazon for the other livestock outside watering troughs.
Adding salt might make animals more thirsty, so go sparingly. In the winters, I'd rather see a little sugar more than salt in water.