Oh no!

I'm very, very sorry about Vim Fuego. He really did die too soon, he was a very handsome boy. And honestly, I don't know if ammonia levels could get THAT high in such a short amount of time... considering that Bettas survive in 1" of filthy water in Wal*Mart, they are a very hardy fish. Unless you did ZERO water changes in 2 weeks, I don't see 8-ish gallons getting dirty enough to kill a Betta so suddenly. How cold do you think it was? I don't really see the cold killing a Betta overnight, either, unless it dipped extremely low. Do you know what the temp was, about? Did you add any new live plants recently? What else did you put into the tank? Did he seem lethargic beforehand?

My guess is that he probably wasn't healthy to begin with. He could have some sort of internal parasite, or he could have just not recieved adequate care before you bought him, or he could have been an older Betta already. Where did you buy him? I would never diss on pet store Bettas, because the one that I recieved lived for 2 1/2 years and was an awesome little CT, but I tend to stay away from pet store Bettas because of all of the unknowns. It just breaks my heart to lose a Betta, and with many pet store Bettas, they're much older than breeder Bettas and have recieved absolute minimum care and kindness in their lives.

Oh, and as for plant recommendations, if you're going to continue planting the tank... I highly recommend Java Ferns! They are available at most fish supply stores (harder to find at Petsmart/Petco, but they occasionally carry "tropical fenrs), and they need no special care. I've had bunches of Java Ferns last for years, with only sunlight, and with frequent water changes. They're also quite cheap, and very pretty. If you want floating plants, I highly recommend Water Sprite, as they're hardy and grow pretty prolifically. For tank bottom plants, Java Moss is equally as hardy as Java Ferns and it looks very pretty, too.