I would have no problem leaving them in an established tank of sufficient size for up to a week or so. 12 days would be pushing it even there. As mentioned above - do you have a ATO? Evaporation, especially in such a small tank, will be just as big of a problem. With a ATO and autofeeder they would be fine. If you have neither, you will need somebody to watch after them for you. What I would not do is go out and get an ATO/feeder, set them up and leave. You will come home to a flood/overfed tank.