The most important thing I have found is keeping a deep enough pan of water for them to be able to wash their whole heads in at least a few times a day. That prevents eye and sinus infections, which can turn ugly!
Regular additions of hot water can work, I've done that.
Right now mine are in the walk-out basement when it's below 20F. Their indoor water stays liquid, as the basement is between 40F and 50F. I did buy a bucket de-icer. I am a very cautious person most of the time, so I read the instructions on the bucket de-icer which talk about the importance of using a GFCI (did I get the letters in the correct order this time?) and four feet of ground rod. hmmmm. Sounds like they want to avoid electrocution. I used the de-icer a couple of times to melt the ice and keep their swim pan (2'x3'x10") liquid. But I removed the de-icer before letting them play in it, as I did not reckon I could get four feet of ground rod driven into frozen stony New England soil. Only one of my ten runners jumped in, by the way.
There are also water heaters that look like large round cake pans upside down. One places the (metal) water dish on top of that. I use large steel mixing bowls.
Both the bucket de-icer and water heater have thermostats that turn them on around 35F and off around 45F.
The cost, depending on where you get them, might be between $20 and $40.