They say that if you can’t walkon the moon, the next best thing is to scuba dive - not because the ocean bed resembles the Sea of Tranquillity, but because it is the closest you’ll ever get to an other-worldly experience. Here you can defy gravity, the light is a lunar blue and the creatures that inhabit the deep are so fantastic as to be far beyond our earthly ken.

The challenge for those who design aquaria is replicating this extra-dimensional strangeness, while revealing its secrets to the widest possible audience. If they get it right, the experience can be wondrous and magical.

But aquaria should also serve a more serious purpose than merely to display fish. As man’s rapaciousness begins to exceed the Earth’s capacity for renewal, and our ability to contaminate continues to wreak havoc on land and sea, an aquarium should educate visitors about the fragility of this precious ecosystem.

The pioneering aquarium Nausicaa, in Boulogne, France, achieves this aim particularly well, with a sympathetic examination of how the fruits of the sea should be carefully husbanded. There, a dazzling prism teeming with tuna is placed next to a life-sized trawler, complete with sound-effects of howling winds and roaring engines. This forms part of the aquarium’s focus on the fishing industry and shows how many interests must be considered in establishing a policy that is fair to fishermen and fish.

With such advances, including those in the much smaller but no less innovative Sea World aquaria here in the UK, the new London Aquarium, in the leviathan of County Hall, has a hard act to follow.

Its statistics are impressive. The aquarium cost Pounds 25 million to develop. It contains thousands of specimens of more than 350 species of fish, from the red-bellied piranha to electric eels, sharks and stingrays, as well as more familiar fare such as trout, mackerel and bass. These creatures live in two million litres of water spread over 170,000 square feet of floor space. It promises all the great waters of the world, and the things that live in them. So what has become of the space formerly occupied by the GLC canteen?

The promotional material contains one exhortation - “dive”. Descending to the aquarium, where dark corridors have the barest of illuminations, the visitor has a genuine sense of falling through fathoms. So it is slightly disorienting when, in the first display, you surface on the banks of a river, somewhere in middle England. Sheep are bleating, birds sing and a church bell tolls sleepily in the distance.

This is the backdrop for a freshwater stream. Most of us are more accustomed to seeing trout on a plate, so it makes a change to see them in a natural-ish habitat. Minnows, dace and loaches also bask among some algaefied green boulders.

It is here that the first of my reservations bubbles up: although there is a legend describing the fish, it is small, with little detail and placed some way from the tank. Some curious pointy-nosed things with serrated spines are not identified at all.

Opposite the tanks, the corridor wall is unadorned, which palls after a while as there is an awful lot of corridor to walk through. But there is, at least, a fresh, fishy sort of smell, thanks to the aquarium’s “aromatherapy”, which pumps “ocean fresh” aromas through the ventilation system.

From the stream the visitor plunges headfirst into the Atlantic, one of the aquarium’s showpieces. This tank extends over three levels, which gives an unrivalled perspective on the fish it contains - wrasse, bullhuss, rays and conger eels (which were staying well in the shadows) and sharks. There is no explanation of this environment though, and as with the stream, the information on individual fish is sparse.

Further on and deeper in the aquarium, tanks become progressively surreal. Carp swim below marble steps and what looks like a bust of Julius Caesar. Further tanks contain submerged bricks, bottles and cartwheels, and even a barn door. This stimulating and intriguing juxtaposition is as baffling as it is refreshing.

By far the most enigmatic setting, and for me the most memorable of all the displays, is that of the Pacific Ocean, the second of the aquarium’s monster tanks. Sand tiger sharks and brown sharks circle colossal, Easter Island-type stone heads, which make a magnificent backdrop to the fish. There are deep, curved seats, low enough for the smallest child and wide enough for several to bunch up together and stare at the noiseless predators less than a foot away.

Other exhibits include a discovery zone and “beach”, where you can stroke the various rays as they undulate past. There is an authentically turbulent tidal tank, with mackerel and huge crabs.

The accompanying electronic music and wave sounds, together with the eerie blue light, enhance one’s sense of well-being and dreaminess. If you dive for real, these symptoms usually presage the onset of something called nitrogen narcosis, but here it is more a sign of one’s equilibrium re-establishing itself.

And so on past the gorgeous pipe fish, the jelly fish and the anemones to the coral reef. Here are some of the most colourful and bizarre of all sea creatures - the dog-faced puffer, the sailfin tang, the black-tailed humbug. The Picasso trigger fish is, as its name suggests, particularly eye-catching. There is a large tank to house the piranha, which are much bigger and beefier than I had expected.

The tropical freshwater tanks lead into a mangrove forest, which attempts to replicate the sights and sounds of a tropical swamp. It has hanging lianas, lush greenery, cascading waterfalls and a great jungle of mangrove roots. To complete the luxuriant vista, the Roman heads have given way to Aztec sculptures and pillars.

Among the inhabitants of these theatrical surroundings are the amazing red-tailed catfish, with whiskers up to a foot long, and the archer fish, so named for its ability to spit long distances and knock passing insects into the water for supper.

It is a spectacular way to end the visit but sadly, spectacle is just about all it is. A great opportunity to educate and inform has been lost. Examples of interactive learning are few, information panels are negligible and there is nothing on the way whole communities can depend on the sea. Industry, conservation, and other environmental issues are not explored.

But these are early days. The aquarium has only just opened and hopefully it will not be long before everyone will know what the pointy-nosed thing with the serrated spine is and where it comes from.

The London Aquarium, County Hall, Riverside Building, Westminster Bridge Road, London SE1 7BP. Tel: 0171 967 8000. Opening hours: 10am-6pm. Adults Pounds 6.50, children Pounds 4.50, concessions Pounds 5.50. Family and group rates available. Talks and tours included. Wheelchair users have free admission; access for wheelchairs is excellent.