Thought I'd share an easy way to mod a 3W existing internal filter for a small, 2G tank without annoying bettas with the strong currents. This might be useful for people who already have a small internal filter and don't want to invest in something else like a sponge filter (or people like me who just don't get how they work).
Step 1: Buy disposable empty soup/broth bags from the supermarket (search Asian specialty shops if you can't find them):
Step 2: Wrap it around the intake with an elastic/rubber band. This prevents the intake from sucking up and tearing your betta's fins (happened to mine):
Step 3: Take out the rain bar, and cut out a small sponge cube in its place. Squeeze the cube between the output and the wall of your small aquarium to create a waterfall. I find that this creates a waterfall way more gentle than whatever I could think of with the rain bar.
Step 4: If you're still worried about the current, use one of these devices (search for timer, socket, plug, power) to have your filter run only in the day so your betta can catch some good rest at night:
And...you have a nice small filter that your betta won't hate!
P.S. Anyone has suggestions on how I should decorate this tank? Haven't had any luck with plants or keeping substrates clean - 2G is just too small to do gravel vacs with! Oh, and ignore the floating plants, work-in-progress :P