Their growth can be stunted incredibly in such a small space, so it's unlikely that they'll get bigger. Does your book say to upgrade tanks every so often? I personally believe that it's much easier to buy a 60 gallon tank instead of upgrading to a 20 gallon, then a 30 gallon, and so on. If you upgrade constantly, it's more expensive and you risk having a mini-cycle each time, even if you add established media.