Hey Lilly,
(Here's a belated welcome to the forum) Sounds like you've been a little burnt with the betta - good on you for doing your homework and coming back to them!
You're in good hands here obviously, I've had success with sponge filters, but find they do a basic mechanical filtration, sort of depends on your tank, and how often you're prepared to do water changes.
From memory you have around a 10 litre tank?
I had a series of these with sponge filters and found this kept the water pretty stable, but they wanted 50% water changes weekly, and a full water change every month or so.
There's a thread somewhere about how everyone cycles their own tanks and the like - might be worth a look.
I add Carbonate Hardness Stabiliser Powder / Almond Leaf extract (or leaf) / Pinch of salt and some shell grit for minerals / Stress coat which is also dechlorinator and if I'm starting a new big tank with an external filter - some CYCLE with bacteria in it.
Since this practise, haven't had much issue with water chemistry.
Oh - and obviously a heater (24-26 degrees C) and plants! I'm big on live plants, what i can keep alive that is - and Java fern + Anubias are pretty easy to keep green!
Best of luck - hey, get a photobucket account so we can see your new purchases.
-Ness