Hey there. Welcome to the forum. I'm relatively new myself but have found it to be a wealth of knowledge. I have just started off my own 5 gal nano. I too wanted co2 so I put in the fluval mini 20g. It's a handy idea but the trouble is the reactor for me was too big and bulky. Plus the refills are a little expensive. I think I paid £14 for three refills. Which doesn't seem too bad but they do run out fairly quick. I also had a few issues with the valve leaking. Fluval replace them so it's not too bad. I quickly upgraded to a refillable setup and just kept the mini 20 for back up. It's way cheaper in the long run doing that. Hope this helps.
I read that you do not have to buy the Fluval refills. Not sure if that makes it much less expensive.
My crypts are producing tiny, constant bubble streams right now...and my brand new (added 02/27/2013) h. glomeratus is pearling. Must be doing something right. Has anyone ever witnessed the crypt airstone? Pretty cool. I tried to get a picture of it, but my phone couldn't capture it...maybe if I got enough requests, I'd go dig out the DSLR.