Rubbermaid brand sells a 20 gallon barrell "Brute" that is a) food grade and b) much more sturdy than the typical plastic trash barrell and c) has almost as airtight a lid as I've ever worked with. 
With any and all storage, you can let it be for some time, but in a few months, you'd want to empty it, clean it with just a clean cloth and reuse. If it gets real slimy etc, you may want to clean with bleach, rinse and ensure all bleach is gone.
Dave
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"Fins to the left...Fins to the Right, I'm going home to play with my reef tonight..."
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Experience in aquaria, 35 years. Experience in marine, 20+ years. Experience in Reef Keeping, 6 years. Always a newbie! | 
