You are aware that it is the aquarium industry that says a shark will be ok in a 180, aren't you?? If they could, they would say you could keep a shark in a glass of water!!! IMO, keep the set-up you had or upgrade to something over 250 gallons for even the smallest shark. Even with the cleanest water, the shark won't have much room to move. It's kind of like you spending the rest of your life in the bathroom. Not much space. That being said, I hope you have sucess with whichever fish you choose for your tank.