Simplify the following text for a Grade 15 native English-speaking person.
Original: As a psychedelic band led by Syd Barrett in the late 1960s , Pink Floyd had moderate mainstream success and were one of the most popular bands in the London underground music scene ; however , Barrett 's erratic behavior eventually forced his colleagues to replace him with guitarist and singer David Gilmour .
Simplified: Pink Floyd had moderate mainstream success and were one of the most popular bands in the London underground music scene in the late 1960s led by Syd Barrett ; however , Barrett 's decline in health would eventually forced his band members to replace him with guitarist and singer David Gilmour .