Just when the thread started getting interesting in the end, we start with this stuff again. I'd rather have seen it totally high jacked and you guys started posting about politics than some kid asking a thousand times of what is the minimum tank size for a shark is. Tank0789, I'm sure you really want to get in this hobby, and I'm sure you can succeed in this, but asking a question until you get the answer you want to hear is a bit much man, you will not like what you hear no matter how you ask it, you will not be able to keep a shark in any tank no matter the dimensions for ALOT longer than you will think. We are talking months before the tank will be stable enough to do anything to. Take the advice that quite a few people has already given you and leave it as that. Don't get me wrong, I don't want to discourage you from doing it. I would love it if everyone in the world were to get into the hobby in some level.
It's almost pointless but I'm going to give some advice, whether you hear it or not. Buy a very small tank, the cheapest you can find. 20g 30g if you already have the 55g use that. Buy atleast 60 lbs of rock, get a skimmer rated for that tank, get sand. Set it up, wait atleast a month, preferably 2. Than get one or two fish, Get some nemo clowns or damsels. Put those in the tank, wait a month, buy a slightly larger fish, ie, dwarf angel or wrasse. And put that in a tank.
And after all that keep them alive for 6 months. If you can accomplish that, than perhaps a shark is right for you, But be forewarned. Buying this initial equipment in and of itself will set you back some money. The tank is the CHEAPEST part of this hobby. Even more if you do a reef. Get some experience under your belt.
Take a weekend scrubbing your tank of algae, let yourself start pulling your hair out with a cyano bacteria bloom. Your fish will breakout with some kind of parasite or disease and that something will effect sharks, even more they are ALOT harder to treat. After that you can graduate to a shark. I would be SHOCKED if you can find someone on here who started out this hobby with a shark.
I've said nothing at all different from any one else on this thread. But you asked for advice, and it's been given over, and over, and over again. Take it, or leave it and save your money.