I don't know what you need for painting the steel stock tank, but there is a spray on product that looks like stone when it dries. I don't know who makes it but you can find it in most paint sections of hardware type stores, even walmart.

There are primers for galvanized metal. I believe aluminum primers work. Once you have the tank primed, you can top it with any paint you like. The stone-look paint is nice, but I really like the look of the "hammered" paint on metal.

You may want to consider using roofing tar to coat the inside of the tank. There are polyurethane coatings available for use on the outside. Without the coatings the tank will last about 10 years, depending upon your weather conditions before rust causes problems.
if using roofing tar it will take some time to dry hard and then you will need to make sure your drain plug can be removed so that after the initial filling the water needs to sit and then be drained out and repeated until no or very little oil sheen is on the surface.
If you want to hide the appearance of the tank you can always build a wooden cedar framework around it as this could help protect the tank from animal damage.