Welcome to the Fish Bowl
A/N: This takes place in an alternate universe where Sauron returned to Valinor with Eonwe to seek the pardon of the Valar. Please note, Aiwendil is more usually known as Radagast, and Curumo as Saruman.
It was all Aiwendil's fault really, Sauron told everyone later. He had been the one to give Sauron the original stock.
It happened once Sauron had been allowed out of Mandos for about a year. He'd been living in Aule's halls, still confined to his room whenever he was not working or under another Maia's supervision. As a result, he'd gotten stultifyingly bored. Books rapidly start losing their luster when they're all there is to do.
So when Aiwendil arrived at his door and knocked, saying "open up! I know you're in there, and I have a present for you," Sauron opened the door and let him in.
Aiwendil was carrying a 5-gallon jar made of greenish glass. It was filled with water, and had a couple of aquatic plants floating in it. Sauron eyed it with alarm.
"Aiwendil, that's very nice of you, but I really haven't your fondness for greenery," said Sauron.
"I know," said Aiwendil, "but that's not the point. Can you show me where you want to put it?"
"Not until you explain why you're giving me a big jar full of pond water," said Sauron.
Aiwendil huffed, and made to place it on top of a valuable treatise on the ideal carbon content of steel for different tools.
Sauron whisked the treatise out of harm's way, glaring. "Okay, what is it?" he asked.
"It is a fish jar," said Aiwendil. "Do you see the little fish swimming around in there?"
Sauron peered more closely at the jar. There were indeed little fish.
"I call them millions fish," said Aiwendil, "because they are very enthusiastic about breeding. The males are really colorful, and the females birth live young."
"Very nice," said Sauron. "But I still don't understand why you're giving it to me. Wouldn't Yavanna appreciate it more?"
"You need a hobby, so that you don't end up murdering Mahtan or Curumo from frustrated boredom. Did I mention that the color patterns are hereditary?"
Sauron paused. It was true he hadn't played with genetic manipulation and selective breeding of anything since Angband. And he was bored. These 'millions fish' might be silly little things – he watched a brightly-colored male as it chased a larger grey female around a stem of water plant - but they were also harmless. Unlike orcs and werewolves, nobody was likely to object to him breeding pretty little fishes.
"You might be right," said Sauron. "Thank you, Aiwendil, I believe I have acquired a hobby.
Aiwendil beamed. "I think you'll enjoy it. This is how you care for them…" after a discussion of required food and water chemistry, Aiwendil was out the door and skipping merrily down the hall, whistling something that sounded like some kind of bird as he he went.
Sauron shook his head as he locked the door, and sat down on the bed. He tapped his finger on his chin, watching his new fish and thinking of what traits he might like to select for.
A/N: Sauron may blame Aiwendil, but I blame my tropical fish habit. 'Millions fish' are more often called guppies.