Rehabbing an old hot water tank

[Robb]

I've picked up an old 80-gallon stone-lined high-temperature solar hot water tank -- the kind from the 80s with the copper heat exchanger built-in. Don't ask me how I got it out of that guy's basement. "Sturdy" would be the delicate way to describe the tank's weight.

There's some corrosion on the connections... I'm thinking I need to replace the nipples and drain valve at a minimum. I know there's a de-scaling product out there that's nice and toxic; anyone know if that stuff will damage the copper coils, or if it's OK?