300 gallons is nothing for a shark tank. A 300 would suit a few bamboo or catsharks for a while, but even those will outgrow a tank of that size. Plus, those sharks are not the most active, usually hiding until feeding time.
If you wanna do sharks, you need at least 1000 gallon tank, and that wont even last you that long if you wanna do sharks that are open water sharks, such as the black tip reef shark. These sharks can get up to 7 feet (females) and males 4 feet.
If i were you, i would do a 300 gallon and 1 or 2 banded bamboo sharks and do some rays. Bat rays, and the blue spotted rays are pretty readily available and are awesome to watch. Plus they dont come with the $1500 price tag that a black tip can cost.
Or how about a 300 gallon trigger tank? triggers get big, there aggressive as hell, fun to watch, and there are alot of kinds. just throwing it out there. Unless your willing to spend upwards of 10 grand for a huge tank and setup, plus the cost of a shark such as a black tip, i would seriously re think what you are getting into.