Double Commonwealth Games champion Proud claimed two gold medals on the first night of finals at the two-day short-course event.
He powered to victory in the 50m freestyle final in a time of 21.60 seconds, having also swam 21.62 in the heats. Proud was the only swimmer to go under the 22 second mark, with his Leander team-mate Tom Fannon coming seventh in 23.02.
Proud and Fannon then teamed up with Juliette Casini and Emma England to win the mixed 200m relay for Plymouth Leander in 1:35.98.
Both Casini and England had also made individual finals on the opening night of the two-day competition.
Casini came fifth in the 100m freestyle in 56.10, with England coming sixth in 56.35.
Other Plymouth Leander finalists included Zirk Kregor, who came fourth in the 100m butterfly (52.56), Freddie Clampett, seventh in the 100 IM (4:32.79), Laura Stephens, who came sixth in the 50m butterfly (27.64) and seventh in the 200m butterfly (2:17.71), Tamir Gonen Cohen, who was sixth in the 200m butterfly (2:16.60), Frederico Poggio, who was sixth in the 100m breaststroke (1:02.77), Eliza Duffy, who took sixth in the 100m backstroke (1:01.74) and Tom Rudd, who came eighth in the 50m backstroke (25.84).