What Aquarium Shark?

Are you referring to freshwater or salt? If you are talking about freshwater then Luiza B is right, the Epalzeorhynchos bicolor (Red-Tail Black Shark) are the smallest of the freshwater sharks available. They only reach 4 1/2 inches but more likely 3 inches. Below is a link to read some info on them, there are many more sites with info on them too.

http://www.aqualandpetsplus.com/Shark,%20Red-Tail....

Now if you are referring to saltwater sharks then your best bet would be a Brownbanded Bamboo Cat Shark. These are in the 7"-11" range and it grows to about 40 inches in the wild, making it one of the smallest sharks available. You need to be experienced to keep these and they need great water quality and proper tankmates, also they should never be exposed to copper. Below is a link to some info on them.

http://saltaquarium.about.com/od/sharkprofiles/p/b...

Hope this is what you were looking for.

Edit: To the_white_stripes... answer; That is great your sharks got so large, you must have taken great care of them, this is probably not unheard of but is definatly bigger than average. Even if they did get 6 or so inches that is still smaller than any ither freshwater shark available. Thanks for adding that though, it was a good thing to mention so the asker can be prepared just in case.

Im not saying that yall are wrong but I have a red tailed black shark that I have had for 3 years and it is 6+ inches. I also had a albino rad talied black shark that I have for 3 years and it was 5 inches when it died the other day. But mine stayed below 4 inches untill i started feeding them shimp pellots. They grow very fast when fed shimp pellots.

Epalzeorhynchos bicolor , I've had one.... grows up to 12 cm

if you get them small enough they'll adapt to your tank size.....buy them to big and they'll just kill themselves from swimming to fast into the sides....it happened to me