Backyard Aquaponics
A place to discuss aquaponics

And old corro tanks may have been lead/tin soldered to join the walls and base...

And they're Zinc coated, because the Zinc is meant to act as a sacrificial anode. It is DESIGNED to corrode thereby protecting the iron. Low pH swings are going to rip it into the water, high swings are going to make it very toxic.
coat it, line it, throw it, do something.