Unknown cleaner - what can I do?

I bought a used 30 gallon food-grade container that was originally used for black tea concentrate. After it was used it was cleaned with some type of unknown cleaner that has a strong smell. It's not a chlorine smell...I do know that.

My question is, what can I used to safely clean my container for brewing use? I've rinsed it and then filled it with water to let it sit overnight.

Suggestions greatly appreciated...

Primary: (empty)
Secondary: Simple mead
Bottle conditioning: American Brown Ale

On Deck: Alpine Spring Clone, Semi-sweet mead

Try oxyclean

Could anything have leached into the plastic and cause issues later?