[SimplySplendid]

I would advise against a Red Tailed Black Shark as they are territorial and aggressive. You should stick with community fish. Also don't put Bettas (aka siamese fighters) in a community tank as they will attack fish that they feel are a threat (such as guppies). Angelfish are "community" fish, but they can be a bit territorial, and also they may well outgrow your tank.
How many gallons is your tank?

Brie

 

[SimplySplendid]

Here are some good beginner community fish:

Barbs (cherry barbs, golden barbs)
Corydoras
Danios
Guppies
Platies
Swordtails
Loaches (Clowns loach, pakistan loach)
Plecos
Tetras (black neon tetra, glowlight tetra, rummynose tetra)
Those are some ideas for your tank.

Fish to avoid:
Paddlefish (can grow to 6 feet)
Irridescent Shark (large & clumsy)
Red talied catfish (can grow to 3 feet)
Pacu (4-5 feet)
South American Leaf Fish (difficult to maintain, need copious amounts of live food)

Hope that helps

 

[SimplySplendid]

No problem.

Brie

 

[SimplySplendid]

Blue, the fish that I mentioned were not fish for cycling/establishing a tank. I posted them as easy-to-care-for fish 8)

 

[SimplySplendid]

Hi Oceane,
I used to have a RTBS and he was fine with all my fish untill one day I put two clown loaches in my tank, and he went crazy. He would chase them around the tank non-stop for several minutes, then rest for a minute or two, then chase them some more. He terrorized the clowns.

So you might have a unique fish or its aggressiveness might not have shown up yet.

Brie