Plastic Jerry cans

One point to be clarified still - we were left with 11 20l plastic jerry cans by the installer - 5 empties but 2 unopened and 3 with pre-mix in them (& one 'rubbish' containing some kind of fluid). I am not clear whether this is because the circuit fluid is less concentrated than expected (the pre-mix is labelled '-18' whereas the certificate says '-14' ) of smaller volume than expected or just that the glycol was over-ordered.