be careful with used barrels.There were some studies(borneman i believe) that showed very poor marine embryo mortality in some species that were housed in certain types of plastic vessels.If anything toxic or even remotely harmful was ever kept in it ,the storing of RODI water will wick some of these substances out of the plastic and into the water.
RODI being devoid of substances tends to be a great solvent and will wash anything trapped in a porous ,non inert container quite effectively.
I had a bad experience many years ago when using a white barrel that was from a car wash.It had the wax or detergent in it that is used for the facility.I scrubbed this vessel many times and pressure washed it also but when I started using it I had very poor health in my corals.I watched the animals degrade for a few months before I removed the barrel.The very next day my animals looked better and looked increasingly healthier each day until I doubted my self and decided to hook up the barrel again.I told myself that it couldn't be the cause.Well the next few weeks showed the very same decline in health in some of the corals.Again I removed the barrel and BINGO,the animals started looking better almost immediately.anecdotal,yes but e3nough for me to learn a lesson.