A FEW months ago, an unusual debate took place at The Regent hotel over the design for Yu, its new seafood restaurant. The aquarium should be bigger, the management said, but the structural engineers thought otherwise. ''If it's any longer than 15 metres,'' they advised, ''the floor might give way''. Those dining in Plume, downstairs from Yu, can rest assured it is management which has given way. The five-tonne, floor-to-ceiling tank - which undulates along one wall of the restaurant - is a safe 15 metres long. The new outlet is the most ambitious project of the hotel's $120 million renovation. The redecorated Harbourside has already reopened and a smaller Plume will follow at the end of the month. Yu will be the first restaurant in Hong Kong to specialise exclusively in live seafood. ''We felt we were ready to do something new,'' general manager Thomas Axmacher said. ''Plume was 14 years old and rather than keeping it the same, we decided to split the space. Hopefully, with two concepts and two markets, both should be busy.'' Yu is due to open on October 10. The giant aquarium, which is the first diners will see when they enter, announces the seafood theme in an unequivocal fashion. The idea had been swimming around in Mr Axmacher's mind for some time. When he first arrived three years ago, he went to a seafood restaurant in Lei Yu Mun. ''I was really impressed by the freshness of the fish and the open tanks,'' he said. ''But the lack of hygiene really bothered me. It bothers a lot of people now.'' Research trips to the United States, Europe, Australia and Japan preceded the final concept. ''We found that other places mixed live and smoked or frozen fish but none specialised only in fresh seafood,'' Mr Axmacher said. The difficulty the Yu team faced was finding fresh fish from unpolluted waters and keeping them alive during transportation. A team of New Zealand engineers came up with the solution. ''They designed the tank and devised a method of purifying and filtering tap water so it replicates deep-sea conditions. They also invented a life-support system so more fish can survive in a limited amount of space.'' The clientele, according to Mr Axmacher, will be attracted to Yu for two reasons. ''First will be those who enjoy fresh fish but have been put off by the health scares. Our fish is guaranteed pollutant-free and absolutely fresh. ''Second will be the younger Chinese who want a change from family-style meals. Here we are offering a hip alternative - a new approach to eating and serving seafood.'' The man responsible for the food is executive chef Jurg Blaser, although his menu may sound limited to those who like the occasional slice of smoked salmon. But within the ''fresh'' parameters, there is a wide variety of choice - of fish and shellfish aswell as methods of preparation. Appetisers include six types of oyster - served on ice, gratinated with herbs or glazed with champagne - two soups, a seafood platter, a salad, steamed shrimps, and deep fried squid. One of the main course options - a basket of mixed seafood - will become apparent before the menu is presented. In the centre of the restaurant, at the top of what used to be Plume's staircase, stands a copper bouillon pot where chefs will poach prawns, shrimps, scallops, abalone, clams, lobster and crab to order. While there are two other a la carte choices - a paella and mussels in white wine - the highlight is Yu's fresh fish menu. This features business-card size paintings of what is available each day, with names written in English, Chinese and Latin. Diners take their pick from the unwitting specimens in the giant tank and then specify whether they want them steamed, grilled, poached, braised or panfried. All fish are served whole, so the tank will always contain a variety of sizes for each specimen to accommodate single or multiple orders. Vegetables and either steamed rice, wild rice or parsley potatoes come on the side. ''It's like going to market,'' Mr Axmacher said. ''There will be at least 10 choices each day, so with five preparation methods that's a total of 50 dishes.'' The fish restaurants of Lamma, Sai Kung and Lei Yu Mun may offer equivalent variety, but because of the language barrier, those with limited knowledge of Asia's piscine delicacies often stick to what they know. Yu offers the chance for experimentation. The learning curve also applies to the wine list. For the first time, The Regent features wines and champagnes available by the bottle and the glass. The innovative decor is in keeping with the seafood theme. As well as the aquarium, there is a glass ''river'' which snakes its way across the ceiling. It has bevelled edges, lit from above with fibre optics to give a subterranean sunlight-on-water effect. The cross beam which used to run along the harbour-facing wall of windows has been removed, giving an uninterrupted view of the water. The idea is the sunlight will bounce off the aquarium and cast watery reflections around the restaurant. Mr Axmacher describes other details with growing enthusiasm: the waiters' silk fish-design ties and the champagne and oyster bar. ''It will be a fun place,'' he says. ''Somewhere lively and certainly somewhere different.''