Unfair Practice

I visited Pick n Pay Express Capricorn few weeks ago to enquire about the process of purchasing gas. I was advised to purchase a brand new tank, which I will have to fill up when the gas is finished , so I bought the tank. 3 weeks later I brought in my tank to fill up as I was advised to do so. I was given back a rusty tank in exchange for my brand new tank. I enquired about this and also reminded the staff member that rust is a health hazard and informed him that I'm living with a toddler who touches everything. Also mentioned that I brought in a brand new tank. I was advised that it's Pick n pay's process.

I was not satisfied with the outcome so I spoke to the manager Issac who also advised that there is nothing he can do it is indeed their process. My question is: Why do you make customers pay for brand new tanks if the same tanks will be replaced with old and rusty tanks two/three weeks after the transaction date? Why was I not informed on the transaction day that, the new tank was going to be replaced with a very old tank when I fill up in two weeks time so that I am at least given an opportunity to make an informed decision? Are they reselling the brand new tanks? I want my tank back, this is unfair practice.