Water Trailer tank for Potable Water - Help/Question

Hi guys... Hopefully someone knowledgeable about water tanks will chime in and help me.

I have a low producing well, so I am starting to supplement having water hauled in to me. Im looking for a water trailer, so I can haul water in myself. I found a guy that has a trailer with a 500 gallon plastic tank on it - its the cloudy clear color tank. It has non potable water stamped on it. He swears he has only used city (potable) water in it. The tank is VERY clean, no algae, etc. In your opinion, is this tank safe to supply drinking water to my family? Of course I would disinfect with a chlorine solution first... Im just curious as to why non potable is stamped on the tank. Thanks for any help, its greatly appreciated.

It might have something to do with the kind of plastic the tank is made of. Some plastics are not approved for potable water containers. Also a darker tank keeps the moss and algae from growing.