I just bought the same tank last month too!
I tried 3 filters already hehe
1. AC 20 - not too noisy but a bit too bulky (and powerful) for the tank, Flow adjustable which is a plus. 3 layers filtering too (activated carbon, sponge and ceramics)
2. Marina internal filter - interestingly I used to think internal filter are quieter but i was wrong...quite noisy and doesnt look good to have a black box in the nano tank (its not huge, but not small either). I unplugged that after a few hrs...
3. AZoo Mignon Filter 60 - A winner for me! I believe it was called Redsea nano something before. I got it off Amazon like 14 bucks. Quite a compact filter (smaller than AC 20), the flow is also adjustable. Quieter than AC 20, I can keep it on overnight after removing the lid and added bubble sheet between the filter and the tank glass. (had to turn off AC20, the noise is audible at night) The only minus is it only has 2 layers of filtering (both sponge), o well there's extra room tho so I just threw in a few ceramic rings
In long run I dun really plan on getting a filter running for my nano...it will be heavily planted so i probably just need a cheap internal power head or a Vuppa (expensive grrr!) to keep water flowing
HTH! I love my 7G cube too!