Which two letters are missing?

A young couple were on their honeymoon and were staying at a hotel with a large swimming pool. They decided to go for a swim, and the bride donned a new bikini that she had recently purchased. As she swam and splashed around in the pool, she soon discovered that the bikini was to large, and the top and bottom kept coming off. As they were the only ones in the pool, she and her husband would laugh and playfully retrieve the bikini from the pool's bottom.
That evening they dressed for dinner and headed to their hotel's elegant restaurant, where they were seated next to a huge aquarium. Strangely, the aquarium was devoid of any aquatic life.
When the bride asked their waiter why the aquarium had no fish in it, he smiled broadly and said, "That's not an aquarium...that's the swimming pool!"