Wayne, it is galvanized, and you have a good point, but we won't be drinking from the tank... Bottled water is cheap, convenient, consistent in taste and quality, and fits in fridge/cooler a whole lot better lol
For those who aren't familiar with the expansion tank mod, this not only gives 2 gallons extra water capacity but it prevents pump from "bumping" or running at short bursts when faucet open partway. It also allows you to run faucet for a moment before pump kicks on so if you just need a rinse, the pump doesn't need to cycle. Great mod and easy to do, coupled with soft pump lines these two will dramatically improve the performance and your level of satisfaction with the water system.
I should do a write up on the mod to break it down. I think everyone who uses their pump should do it! Lol