20g tank, been a while

[popcornadict]

So I got a 20 gallon tank for free - 30"x12" approximately, so it's the long skinny one. It's been a while since I've had a real fish tank (not including my two betas, which I've surprisingly kept alive for more than a month now) but I was wondering if anyone had any recommendations as to the filters or heaters, if there is a preferred brand, and what are everyone's thoughts on under-gravel filters? Also, I'm planning on going with a fairly mono-chromatic color scheme as far as decoration goes - black gravel, with a little dark red mixed in. What kinds of plants does everyone recommend? The tank is only about 12" tall, and I don't want anything that's going to be bent over because it's too tall. I'm planning on bala sharks for sure, but am also wondering what will work with them. I've had them before, but it was only a ten gallon tank (apparently why I've never been able to keep anything alive for more than a few weeks at a time) and that was all I had in it except a pleco and a couple mollies. (They got along just fine - the mollies were actually more aggressive than the sharks were.) I'm hoping to find some interesting fish in reds to contrast with the black and white. Suggestions? Also, should I also have a bubbler, or is the flow from the filter enough, should I go with the over the back of the tank style? I've heard conflicting opinions on that.
Thanks in advance for everyone's input!
-P

 

[popcornadict]

Okay, so after browsing some fish store websites at another member's advice to another member's question, I've found some fish I like - bala's are still on that list, but then I went to my local fish store and did some looking. Most of what I like are tetras. Here's what made the narrowed down list:
Marble Hatchets
Head & Tail Light Tetras
Black Moscow Guppies
Bleeding Heart Tetras
Silver Dollars
Serpae Tetras
Pristilla Tetras
Red-tail Shark
Octocincus (spelling?) Pleco
Black Fin Tetras
and I abbreviated this, so I think it's Green Face Rams? (I abbreviated as "G. Face Rams")
The "pet store" specializes in fish, but when I asked the guy what his thoughts were, he said that aside from the back wall, which was entirely African Cichlids, and the alcove, which was salt-water, I could put pretty much anything I wanted together with a 20 gallon tank. Not a lot of help there. I'm still debating. I like the reds and black and white/silver for fish colors. What are everyone's thoughts on what to put in the tank? I'm thinking about going with sand instead of gravel, but I'm torn because I really want to use an under-gravel filter.