| I'm sorry Keith! I know, everytime we just want straight answers we get like 50 opinions. But what I meant to say is "absolutely not" submersible! You can use it to that water line, and it'll probably work wonderfully for a long time. I still actually have a 200 W jager in my prop tank; i just didn't submerge it fully so it works well.
the one you bought is actually fine for your 12G nano (I'm assuming it's the JBJ nano cube). you might even have been able to get away with a 50W too. You can run it in the back return pump chamber so it's out of sight and not fully submerged. I think it'll fit.
good luck!! | 
