Hi Aloha13,
I have been researching this very thing and also don't want to pay the high price for stock tanks. Another issue with them is that you need to treat them like pots with the type of mix you use for pots becuase they have no cantact with all the goodies in soil. I am purchasing galvanized steel window wells (like you see around basement windows) and bolting two together to make a planter. They are reasonably priced, come in different sizes and remain open to the ground below. Hope this helps

Some preparation is required prior to filling the tank.

Zinc, drainage issues

finally found a pic of what I'm doing...the wells come in various sizes and also rectangular

Here is a link that might be useful: http://greenwalks.wordpress.com/2010/04/21/sweet-cylindrical-stock-tanks/

I wonder if the zinc leaching issue would apply over the entire life of the tank versus decreasing or ceasing over time? I grew walla wallas in it the first year I had mine; they did OK, but not quite as well as I was expecting.

I have a zinc coated stock tank with Goldfish in it. No problems after ten years.
Mike

I just installed a galvanized fire ring (which has no bottom) to be used as a planter, 3' diameter and 1' high.

I'll try to post my results later in the growing season.