I'm on a look out for safe large containers too. Plastic totes are so inexpensive and universal, as are plastic barrels cut in half, but I'm concerned about the plastic. So I looked into those stock tanks.
I've seen them described as both safe and unsafe. They are zinc plated and there is something about the plating process and the process to clean and prep the base metal prior to electroplating. I vaguely remember if there is nickel involved then it won't be safe. There could be some lead involved in the process. Maybe cadmium? (This is from uncertain memory)
It may depend on where they are made. Stock tanks maybe different from the smaller wash tubs, too, which are often available cheap from craft store.
For my part, one issue that has me not quite ready to try using one is that I did try a washtub (that was not quite as cheaply made as the craft store ones made in China) to use as an in-ground tiny pond. It rusted after a couple of seasons, killed the fish, and started to float rusty oily goo, so I had to dig it up and get rid of it.
Remembering that really made me wonder if galvanized metal containers would be safe to use as planters especially since you have to punch drainage holes in them which would compromise the rust-proofing galvanization and -- I imagine -- speed the rusting process. And you can't see all that going on inside, filled with growing medium/soil mix.
Trouble is that MOST of the glowing instructables and blogs about using stock tanks as above ground ponds and planters only talk about how they set them up and planted them, then how they fared during the first and maybe 2nd year at most.
What have your research turned up?