American superstar Caeleb Dressel produced more fireworks Saturday as he romped to a hat-trick of gold medals, taking his tally to six on a dark day when a fatal nightclub accident shook swimming's world championships. Day seven also threw up further doping controversy with Australian officials announcing freestyle relay world-record holder Shayna Jack had tested positive for a banned substance, explaining her mysterious no-show in South Korea. But Dressel, swimming's squeaky-clean, tattooed glamour boy, helped refocus attention on the pool as he retained his 50 metres freestyle and 100m butterfly world titles in style, before leading the American mixed 4x100m freestyle team to a new world record. "It doesn't just come together by accident," said Dressel, after becoming the first swimmer to win at least six gold medals at two world championships. "It wasn't easy but I don't want it to be easy, I really don't." Bouncing on his toes like a prizefighter before the 50m free, Dressel exploded from the blocks and never looked back, clocking a championship best 21.04 seconds, comfortably clear of Brazilian Bruno Fratus and Greek Kristian Gkolomeev, who tied for silver in 21.45. Dressel, who scooped seven gold medals at the 2017 world championships in Budapest, let out a roar of delight but had very little time to savour the moment. After the shortest of breathers, the 22-year-old was back for the 100m butterfly final and after a long, hard stare down lane four, he got it done again. - World record - Once again making his signature fast start, the American touched in 49.66 -- the second-quickest swim ever, a night after shattering Michael Phelps's 10-year-old world record. Russia's Andrei Minakov took silver a full body-length behind, with South African Chad le Clos third. Just when his rivals thought it was safe to get back in the water, Dressel gave the United States relay quartet the lead they would keep, Simon Manuel bringing the Americans home in a world record of 3:19.40, just over half-a-second in front of Australia. "I'm glad it's over," said Dressel, who also took a silver in the 4x100m mixed medley and could win a seventh gold in the men's 4x100m medley relay on the final day. "But tomorrow I will be ready to swim one more, and one more fast." Dressel's heroics came after the shock news that two locals died and eight athletes were injured after an indoor balcony at a Gwangju nightclub collapsed in the early hours Saturday, sending party-goers plummeting over five metres onto people below. A doping controversy followed when it was revealed Jack had failed a drug test, a development that could not have come at a worse time for Australia, after Mack Horton's highly public podium snub of China's Sun Yang on day one. - 'We stand for clean sport' - Horton refused to step on the podium in protest at claims made in a leaked FINA doping panel report that Sun had allowed blood vials to be smashed with a hammer after testers visited him last year. "I shouldn't have to put up with these insults," Sun, a three-time Olympic gold medallist, said later, insisting he was standing up for all athletes by refusing to let "unlicensed" testers take samples. "I'm protecting myself and all athletes, I have nothing to prove." Aussie sprinter Cate Campbell chose her words carefully when asked about Jack, who has denied knowingly taking a banned substance. "We stand for a clean sport," said Campbell. "I think the fact that Shayna isn't here strengthens that stance." Others were more forthright, le Clos sniffing: "If you test positive, you shouldn't be allowed to swim." Meanwhile, American great Katie Ledecky displayed a champion's heart to win her fourth successive 800m freestyle gold after being pushed to the limit by European champion Simona Quadarella. Ledecky, who withdrew from her 1,500m title defence and 200m heat through illness, held off the Italian to win in 8:13.58 and capture a 15th world title. Elsewhere, Sweden's Sarah Sjostrom swept to victory in the 50m fly in 25.02, becoming the first woman to win three world titles in the event. Caeleb Dressel has won six gold medals at the world swimming championships. USA's Katie Ledecky won her fourth successive 800m world title.