Copyright 1990 The Chronicle Publishing Co. The San Francisco Chronicle DECEMBER 22, 1990, SATURDAY, FINAL EDITION SECTION: DAILY DATEBOOK; Pg. C4; SOMETHING ELSE LENGTH: 799 words HEADLINE: SOMETHING ELSE BYLINE: JOEL SELVIN BODY: JELLYFISH, THE MTV FAVE San Francisco's best new rock band, Jellyfish, went right into MTV's rotation this month with the release of the second video, ''That Is Why,'' off of the band's debut album, ''Bellybutton.'' Another track will be used on the sound track of the next John Hughes movie, ''Career Opportunities.'' The band recently recorded the quirky little signature instrumental MTV plays at the top of every hour (as a Jellyfish floats across the screen). 'Fish recently completed a successful club tour with a knockout show at Haight Street's I-Beam, and is laying out until a showcase date during the Gavin Convention of radio programmers in February. GRAPHIC: PHOTO,Jellyfish, another product of San Francisco's rock scene getting heavy rotation on MTV