He has her agree to be obedient.

Before he spoke, he wrote in the Note, "When Aquarius agrees to the rules Natsu Dragneel sets out for her, she will follow them to the letter, and get a rush of pleasure whenever she follows a rule."

"From now on, refer to me as Master," Natsu told Aquarius as she flopped. "And you will do whatever I say. And from now on, you will treat Lucy with respect. And you will agree to these rules. Off the hook."

"Fuck you," she growled, before flopping again as a result of her disobedience.

"Each time you refuse, I'm going to wait an additional minute before I say Off the Hook again," Natsu said, pulling Lucy onto his lap, making her giggle happily as she grabbed his hands and guided them to her boobs.

"Go to hell," Aquarius stated, before flopping again.

"...whoops, I said you know what when talking to her there," Natsu shrugged, playing with Lucy's tits as she cooed. After a bit, Natsu said again, "Off the hook. Do you agree yet?"

"Never," she grunted, and once again flopped.

"She's like, stubborn, always has been," Lucy giggled as Natsu kept groping her breasts.

"Maybe, but she hates this fish thing," Natsu replied, tweaking Lucy's nipples and provoking a squeal from her.