Well got me a 55g barrel it was used for glue like a envelope has on it i used vinagur and hot water to clean it think ill be ok? Its a blue plastic one
I wouldn't use any barel for water for my tank that had been previously been used for anything with a chemical base.
You could use a 5:1 water/muriatic acid soak to potentially get rid of any residue that may be left. Follow that with SEVERAL fresh water rinses.
A better and potentially easier bet would be to find a barel that was used for something water based.