I had a pair of 6-line wrasses that would do the same thing... I'm pretty sure it was on purpose, every few days I'd tear everything out of the sump to rescue them, they would jump over the overflow barrier like they knew where they were going.... how they survived in there I'll never now... three unprotected return pumps, and two skimmers, I can't think of a worse place for little fish. once I figured out how they were getting down there some plastic canvas solved thier adventuring.