My advice to you:
1. Rather go to uni and enjoy it doing a useful degree or a degree that could help you in your intended line of work, i.e. marine biology, business management, biology etc.
2: While studying this, get a part time job at a place like a pet shop, animal shelter or anywhere that deals with animals sick and healthy 24h. Not open 24H, but have to look after them 24h/365.
3. Carefully think about this weighing pros and cons. Remember you are 18. A pet shop means no popping off to London or festivals at a whim, you WILL ALWAYS HAVE TO PREPARE, WAY IN ADVANCE.
4.If you open say a bookstore and you buy a book that isn't selling too well, it sits on the shell. If you by a fish that isn't selling too well, you still need to feed it, clean the tank and keep it healthy, that all costs money. Your store is chewing money at ALL times as there is no such thing as the store is closed. The pumps are running 24/7.
5. I think it is great you are asking these questions and I think you should go for it (one day at least). I am by no means trying to discourage you guys, just playing devil's advocate with regards to restraints.
Just my 2 cents mate.