Realistically, a tank shouldn't be that much cooler than a t-shit, it's just missing the sleeves. But we all know that for some magical reason, it can be 1000 degrees out and as long as you're in a tank, you'll be comfortable. That being said, you should probably check out the Vans Beeston Horizontal Tank Top. The Beeston is made from 100% cotton and features a contrast chambray pocket so you know it's cool.
Contrast chambray chest pocket