We know because we know what they have been made of in past sales. These "metal" tanks are not the same as the SS ones which were stainless steel 
There is a LOT more info about stuff like this over in the ask questions thread at the other place though and feel free to ask whatever you need to know, if we know we are more than willing to help or sometimes you can even get an answer out of Dino or Jon on things we dont know
There is a LOT more info about stuff like this over in the ask questions thread at the other place though and feel free to ask whatever you need to know, if we know we are more than willing to help or sometimes you can even get an answer out of Dino or Jon on things we dont know
So, now how do you know the names of these? Or are you talking about what their made of?