If you're changing liquids really frequently, probably a tank isn't the best bet -- tanks shine when you have one or two liquids that you want to vape a lot of.
Perhaps just pick up some mega/jumbo cartos, around 1.5 - 2 ml, dedicate a carto to each liquid. Even some standard size 1 ml cartos are good if you're really changing things a lot. Right now I vape almost 6 ml/day, doing well with simple 1 ml cartos -- I leave the top rubber piece off, to refill easily.