Have her add her mouth.

"Wow little fishy, your tits feel great," he moaned.

"S-shut it," she mumbled, looking away with a blush again as she bit her lip.

"Hey now, no need for hostility, I know, how about you add your mouth to it?" he asked her, giving a sly grin.

"...f-fine," she huffed, turning her head back as she kept working her boobs on his shaft, and licked his tip, looking disgruntled, but still working at it.

"Good fish," Natsu told her.

"...unh..." she mumbled around her tongue, before sucking on his tip, squeezing her tits together around his shaft as she kept going.

"Oh, you, like, managed to tame her?" Lucy giggle as she walked in, still wearing nothing but an apron.

"Yes, yes I did," Natsu moaned, pumping his hips, and his cock started to throb, before he grunted and released his seed. "Swallow," he told Aquarius, who reluctantly obeyed, though she still had some embarrassed disgust on her face.

Natsu grinned, and got ready to fuck her, before running into an issue... No legs and no pussy... Huh... Well, he could use the Note to give her a human form... Or just a human pussy where scales met skin, she was hot as a mermaid, even if she was a bit bitchy... Decisions, decisions...