Siobhan-Marie O'Connor and Sophie Allen claimed women's 200m individual medley silver and bronze respectively, while Jemma Lowe finished third in the 200m butterfly to complete a successful first day for Great Britain in Denmark.
But there were no medals for the team as they returned for day two, despite competing in four finals.
Halsall was Great Britain's best individual finisher as she touched home fifth in the 100m freestyle final in 52.50 seconds.
The 23-year-old then returned to the pool alongside Chris Walker-Hebborn, Michael Jamieson and Lowe for the 4x50m medley mixed final, Great Britain finishing fifth.
Walker-Hebborn was also competing in an individual final as he finished sixth in the 50m backstroke showpiece with a time of 23.59, Craig McNally having finished 38th.
The other Brit to contend a final was Allen, but she was unable to add to her bronze medal as she finished eighth in the 200m breaststroke final, touching home in 2:24.51 minutes – Hannah Miley having failed to reach the final, finishing 16th.
O'Connor is in line to add to her medal tally though as she heads into the 100m individual medley final as the fourth-fastest qualifier, coming second in her semi-final with a time of 58.90 seconds.
Jazz Carlin and Miley also advanced from their respective 800m freestyle heats, although the latter was not able to repeat the feat in the 50m butterfly.
© Sportsbeat 2013