Zinc from new galvanized metal can sometimes leach into soils at levels high enough to inhibit good plant growth. An old metal stock tank would probably be fine, but a new one would not make a good planter unless you lined it with plastic.
The older tanks are the very best in part because mose of them do have holes in them.. farmers usually are greatful for anyone wanting to take off their hands a leaky watering trough. If you get one that has no holes.. be sure to add them if not you will end up with a soggy waterlogged mess.. and a very heavy trough when it comes to trying to move it etc...
An old used stock tank has worked very well for me to actually get ripe tomatoes in my zone 5 local. It warms up faster than the ground. I have been able to move it a couple of times to find the best sunny spot over the 4 years I have used it. I can easily throw a sheet over and tie a string around to hold it, to protect from freezing.
It has the added benefit of having been free for hauling it off from the previous owner. The bottom had rusted through lots of tiny pin holes. It holds water long enough for the plants to take up what they need, but will not drown them in a heavy rain. It holds 6 tomato plants and only takes about 2 five gallon buckets of water twice a week during hot weather. I love it and will look for more.
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