I, and some others, use an aquarium heater:
https://www.backyardchickens.com/articles/aarts-heated-waterer-with-horizontal-nipples.67256/
@blackdog043 ,and others, use a trough/stock tank heater...
HN's are the way to go!!
I don't use anything else except for open waterers for electrolytes during heat waves.
Here's my thoughts on 'nipple training'.
First, it's good to know how much water your flock consumes 'normally', I top off water every morning and have marks on the waterers so I...