Backyard Aquaponics
A place to discuss aquaponics

I got 6 of these barrels at $20CDN each.

Are they safe to use for aquaponics?

How should I go about cleaning them out, other than dish soap and water?

I believe that is a type of fertilizer. As long as it isn't any harsh or hazardous chemical containers it's fine. For cleaning, I get a new sponge from the dollar store. Rinse them really well and scrub them down. No soaps or cleaner or any thing. Normally I check for suds while rinsing because that would let me know if the barrel held soaps or sufficcants. Flush thoroughly until the water runs clean.