Whew! That's a ton of water for 7 chickens to change twice a week! You might consider downsizing to 2 or 3 gallon buckets and save some wear and tear on your back

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So last winter, I did this:
https://www.backyardchickens.com/threads/not-enough-water.1505012/page-2#post-25239720
We have more chickens, now, so this year I increased capacity to 3.5 gallon buckets AND found translucent ones so I can monitor the water level. In the cold weather, I don't change the water so often - just give a good rinse and refill when needed. I also switched heaters, from painted aluminum bird bath deicers (from which the paint had begun to peel), to a silicone type. I like bird bath deicers because they are low-wattage and don't cost a lot to run. Many stock tank heaters are 250W or more, vs 60-80W for a bird bath deicer. If you're just keeping a few gallons from freezing, that's all you need. On a few sub-zero mornings, I've had to break a little ice off the outside of the poultry nipples, but that was on very cold mornings!
Here's what I ended up with this year:
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You can see that there are some black Gorilla tape squares - inevitably, cracks happen - usually at the top and/or the bottom of the holes. I've repaired my other buckets several times with 100% success, but it's a pain when they leak to have to empty them, dry the area, apply the tape, refill . . . . So this time I just started off putting tape where I know cracks/leaks tend to happen.
I really like that these buckets are transparent - I can easily see when water levels are getting low. They're a little heavier than my nice little 2 gallon buckets, but I don't have to fill them all the way to carry them out to the run, and can bring a couple of old gallon jugs out with me later to top them off later if I want. I'm getting old and I just don't feel like carrying that much weight, lol.
For cord access through the lid, I drilled a hole just inside the edge of the lid, and then snipped from the edge to the hole. By pushing the snipped slit apart, you can slide the cord to the hole. The hole also acts as a vent hole so water can come out as needed.
So - these new heaters haven't had to prove themselves in sub-zero weather yet, but we've had some 20's so far, and the heaters are working. When I put my hand on the sides of the buckets, they're slightly warm to the touch, so I'm hopeful they'll do well when it gets really cold.
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I bought the buckets here:
Home Depot 3.5 Gal. Translucent Gray Bucket
And the Bird Bath Deicers here:
Bird Bath De Icer, 60w Submergible Utility De Icer, Thermostatic Controlled Winter Water Heater Deicer for Bird, Chicken, Duck, Farm Trough
Hope that gives you some ideas for what might work for you!