I am having some of this problem myself. Not integral tanks, but epoxy FRP tanks. They were internally painted with an epoxy supposed to be certified for drinking water, but still the water has a taste if it has been in the tank more than a few days. I never found a tank coating certified for small tanks, yet clearly they exist as many small well tanks are "epoxy lined". 1000 gallon sphere has a surface/volume ratio of about 1/ft, while a 30 gallon sphere is about 3/ft, so at least three times as "contaminating".