The double Olympic champion was one of 13 British medallists at the event, clocking a championships record 8:19.86 to win the 800m freestyle ahead of Spain's Mireia Garcia Belmonte and British team-mate Keri-Anne Payne, the world open water champion.
“I’m very pleased with that swim,” said Adlington.
“To be honest I never expected to finish in that sort of time given the training I’ve been doing at this stage of the season.
“I swam here two years ago and went 8:21.00 off the same kind of preparation so to post that time is a bit of a shock.
“I haven’t backed off this week and am training right through which makes this kind of performance even more surprising."
World silver medallist Hannah Miley won the 400m medley, again ahead of Belmonte, while British team-mate Aimee Willmott claimed third.
Scot Miley then secured her second medal at the Sant Andreu Club Natacio when she won bronze in the 200m breaststroke.
Chris Walker-Hebborn posted his fastest time of the season to take gold in the men's 200m backstroke, clocking 1:59.04, while James Goddard swam a controlled race to win the 200m medley, Georgia Davies claimed the 100m backstroke and Michael Jamieson dominated the 100m breaststroke.
David Carry, who is looking to secure his place on Team GB at this month's ASA Nationals, the second chance trials, in Sheffield, looked in good form as he won 400m freestyle silver behind France's Sebastien Rouault.
Elsewhere, Roberto Pavoni and Matt Johnson claimed silver and bronze in the 200m butterfly, Jazmin Carlin finished third in the 200m freestyle and Jemma Lowe touched for the final spot in the podium in the 100m butterfly.