ZPL Birthday Bash alum Aaron Carter was found dead in his home in Lancaster, California on Saturday.  He was only 34.  Aaron’s house sitter found his body in the bathtub around 11:00 a.m. and called 911.

There’s no word yet on the cause of death, but authorities say they found no evidence of foul play. 

After selling millions of albums in the late ’90s and early 2000s, Aaron struggled with substance abuse, and went to rehab several times. 

His most recent stint was this past September, after he lost custody of his son Prince, who will be a year old later this month.

Prince’s mother, Melanie Martin, issued a statement saying, “We are still in the process of accepting this unfortunate reality.  Your thoughts and prayers are greatly appreciated.”

Melanie referred to Aaron as her fiancé . . . it seems like they were kind of on-and-off.

Aaron’s older brother Nick Carter of the Backstreet Boys said, “My heart has been broken today.  Even though my brother and I have had a complicated relationship, my love for him has never ever faded.”

He added, “Addiction and mental illness is the real villain here.”

Nick was onstage with the Backstreet Boys last night at the O2 Arena in London, and the band paid tribute to Aaron.

Hilary Duff, who famously dated Aaron when they were teen stars more than two decades ago, said, “I’m deeply sorry that life was so hard for you and that you had to struggle in front of the whole world.

“You had a charm that was absolutely effervescent . . . boy did my teenage self love you deeply.  Sending love to your family at this time.  Rest easy.”