Emma Eaves won two medals in personal-best times at the national age group championships in Sheffield.
The 11-year-old Brighton Club swimmer claimed a silver medal in the 100m front crawl (1min.4sec) and bronze at 200m (2min.18sec).
Her coach Nigel Carracun said: "You can't ask any more of swimmers than to swim personal bests. The 200m swim was a great effort as it knocked four seconds off her previous best."
Ben Hutchinson, from Crawley took a bronze medal in the 15-year-old boys' 100m freestyle.
Carracun, who also coaches Sussex, said: "Overall it was a little disappointing for the county.
"But the meet combined two year age groups so the competition was that much tougher.
"Also it was decided to do away with the shorter events in which Sussex had a lot of success last year.
"But we have some talented swimmers in the county so hopefully we'll come back stronger next year.
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