"I think I'd like to merge with myself now," Lisa said. "I really want to know what being a waterbed aquarium is like."
"Sure, go right ahead," said Ty, standing.
Lisa, that is, human Lisa, disappeared, which was not what Ty was expecting. She'd merged her human form into her waterbed/aquarium form. But then waterbed/aquarium Lisa vanished, and human/aquarium Lisa reappeared. Ty was pleased to see that the latest changes he made worked properly, as her smaller form was not overwhelmed with too many fish, rocks, plants, etc. She was back to her previous complement.
"Oh, Ty!" she said, "That was amazing! Even better than mattress topper form! When in human/aquarium form, I can feel the fish swimming around inside me, and it feels nice. Well, except when that one naughty fish pokes my nipples."
"Want me to fix that?" Ty asked, as he reached out and mimed poking her nipples, though he didn't actually do it.
"Maybe later," Lisa said. "But in waterbed/aquarium form, having so many more fish inside makes it feel... more than just nice. It's wonderful! And even better combined with feeling both your and my human body sensations as we fucked. I want to split and be a waterbed every night! I know we talked about my not spending too much time in an alternate form, to avoid mental changes, but if I merge every day, that would be OK, wouldn't it?"
"Yes, I think that should be fine," Ty said. "I just think the week that one of yourselves spent as my cock was far too long, but you've demonstrated many times that being in another form overnight doesn't hurt you."
"Thank you, Ty!" She kissed him. "And can I be your cock, overnight, too?"
"Sure. It's getting a bit late," Ty said. "Shall we go to bed? Aquarium waterbed?"
"Yes!" Lisa said. She made two duplicates of herself, one directly into Ty's cock, and one directly into her new waterbed/aquarium form. They lay down upon her again, and fucked again, before going to sleep.