You might want to paint the inside of the tank with a fish-safe epoxy paint, just to make sure. Or, line it with pond liner. Steel stock tanks always rust eventually.

Temperature may also be an issue in TX. Since most stock tanks are only a couple of feet deep, the fish can't escape to a cooler area. Monitor the temps and consider painting the outside of the tank white. Shade cloth might also be handy.

Sue

Yes you can, tilapia are a tough fish and can stand tough conditions; however, they can't take water below approx 70F.
The more you stack in the tank the more attention will be required in the form of water cleaning and addition of O2.
Make sure you have a net or wire on the top so they wont jump out.
They grow fast and are very good to eat.
Some states require a non-native species permit.
Lynn

I have been hearing some good things about Blue Max Liquid Rubber Membrane for use in cisterns and ponds (potable drinking water grade), so I think it would be perfect for lining a galvanized tank for fish.

Also investigate Backyard Aquaponics, where the fish-poopy water is pumped into plant beds (decorative or food types) to feed the plants, the water is cleansed by the plants and pumped back into the fish tanks.

Let's see if they'll let me post the link...

Sue

Here is a link that might be useful: Backyard Aquaponics