the nano cubes aren't bad little tanks...but they are little. its too bad you do not want to go with something bigger as you will find it to be much easier, but a small tank is doable. if you want coral, etc you will need more pumps than just the one built in to a nano cube otherwise you should be ok, though for algae growth reduction a powerhead wouldn't hurt. my biggest suggestion is to read the posts in this forum about setting up a nano and also read a couple good books (Paletta's "New Marine Aquarium" is an excellent starting point) lest you get too deeply involved in this (financially and psychologically) and never have success
both of these posts are worth reading:
The Darkside :O Update: 15gal Via Aqua