Whoa whoa whoa slow down. The sharks you described are NOT salt water. They are freshwater. And they AREN'T really sharks. They are Cyprinids. Bony fish, osteoicthys! Not cartilaginous. Live sand is for SW live rock is for SW. Let's start all over here. When you say shark what names are you thinking about?
Additionally, no one is going to sell you a salt water shark unless it's already half dead in some unscrupulous dealer's tank with only a 55 gal. Saltwater sharks are one of those thigns that belong in the ocean...not the aquarium.
edit: okay after parsing that closer...you had 6 black tip reef sharks...a shark I've caught fishing in galveston...a shark that gets 6 feet long...a shark that I pulled up that was 3' long....in a 55 gallon tank. I'm sorry but no, you cannot have a shark of any kind. Multiply your tank volume by 10 or 100 and we'll talk.
I really don't have sympathy for someone who wants to keep sharks. They are cool. But they are cool because they are in the ocean.