As Tikibirds said, you won't be charged the moment the auction ends, rather, after you win you'll get an email or notification informing you. It will then offer a link to "pay now". This is where you double check your address etc and card number. That's when the money will come out (whether it is withdrawn immediately depends on your bank, some can take a business day or two to clear the transaction). It's typically only after the payment has been approved that the sender will post your goods. If you ignore the Pay now link, or wait too long to pay, the sender has the option to cancel the sale and re-auction the item. (Most will send a message first, something like payment overdue, please pay now etc).
The good thing about PayPal is it offers buyer protection, in case the goods never arrive, or in case your fish is DOA (hopefully not) and the seller won't give you your money back.
Sorry for the long post, I spend A LOT of time on eBay. :D