Your hearing is correct and your son won't be disappointed. For a city in the middle of the desert, Las Vegas is home to a great number of fish and other water creatures, many of which can be viewed for free.
The Atlantis Aquarium to which you refer is located in the Forum Shops and is a 50,000-gallon saltwater tank that's home to about 500 fish, including rays and sharks. Twice a day, one brave diver ventures in to feed them (in the case of the sharks and rays, by hand). The feedings take place at 1:15 pm and 5:15 pm daily and, even cooler, at 3:15 pm daily you can enjoy a free "below-the-scenes" tour, which takes you beneath the tank and through the filtration system, kitchen, ray and shark pool, and fish acclimation areas. Meet at the tour-information sign in front of the aquarium, or call 702/893-3807.
Shark Reef at Mandalay Bay is open daily 10 am-11 pm (last admission sold at 10 p.m.) and costs $15.95 for adults and $10.95 for children 5-12. The aquarium holds nearly two million gallons of water and includes a tunnel that you can walk through, as sharks swim overhead. Other exhibits include rare reptiles, mezmerizing jellyfish, and a petting tank where you can tickle Southern rays, Port Jackson sharks, and Horseshoe crabs, plus there's a staff of 16 naturalists to answer your questions. If you happen to be at Mandalay Bay, but don't want to pay that (rather hefty) admission price, there's also a 12,200-gallon aquarium in the hotel lobby.
Talking of hotel lobbies, the Mirage has a 20,000-gallon saltwater aquarium located behind the front desk, where anyone's free to take a peek. It's home to angelfish, puffer fish, tangs, and more than 1,000 coral-reef animals (representing 60 species) from Australia, Hawaii, Tonga, Fiji, the Red Sea, the Marshall Islands, the Sea of Cortez, and the Caribbean. The tank is 53 feet long, eight feet deep, and six feet from front to back, with an interior carefully created to simulate a coral reef, without using any live coral (now a threatened species itself.)
Another excellent free exhibit is Silverton's collection of fresh and saltwater aquariums. The 117,000-gallon artificial reef is home to over 4,000 tropical fish, plus three species of stingrays and six species of sharks. Three times daily there's an interactive feeding show (1:30, 4:30, and 7:30 pm), with a Q&A portion, where you can pose questions to the diver. The Mermaid Lounge bar, adjacent to the aquarium, also features two 500-gallon LED-lit jellyfish tanks, which look like those hypnotic lava lamps from the '60s. And finally, the Bass Pro Shop is also home to an 18,000-gallon water feature, housing over 100 koi carp, a 20,000-gallon Nevada lake display that's home to huge channel catfish and other freshwater species, and a 28,000-gallon rapid-flowing canyon display, housing water birds, turtles, and more fish.
The newest addition to the Las Vegas' aquarium scene is the Tank at the Golden Nugget. While the pool area and water slide that goes through the middle of the shark tank is for hotel guests only, the 200,000-gallon aquarium, with its collection of sharks, game fish, and other aquatic creatures from around the world, is viewable by all for free.
The only other aquarium we knew of in town was originally outside MGM Grand's Sea Blue restaurant, but has since been removed. The reason? Apparently, the birds that occasionally make their way uninvited into the casino, via trade entrances and other ingresses, were crashing into the glass tank. This came as a surprise to us -- we've encountered birds in Parisian restaurants and taking a free ride on the London Underground system, but never in a Las Vegas casino. If any of our readers has ever encountered a bird (of the feathered variety) inside a casino here (and we're not talking about roasted chicken), we'd be fascinated to hear from you.