Plastic water tanks, including flex tanks, should be NSF (or FDA) *rated* for usage with Potable water. If not so rated it means a strong possibility that the plastic isnt compatible vs. 'leachables' (large unbound polymer 'fragments' that continually get into the water) ... not good for human liver nor kidney nor brain, etc. function.
Even polyethylene water tanks should be NSF 'rated' for potable water to insure that its 100% PE and the PE hasnt been loaded with an unknown 'filler'.