Unfortunately, a 300 gallon tank is usually only big enough for one shark, and not multiple sharks. There really is nothing special you need for a shark tank though. The most important part is having the size and room for the species of shark you are looking to keep. Other than that, the usual equipment applies: large sump or wet/dry, huge skimmer, powerheads to move the water, heaters, rock to put in corners, as well as sump (for extra biological filtration - since you cannot have much rock inside the display aquarium).