John stared morosely at the cup of milky tea he held in his hands, swirling the liquid gently as he sat on a park bench. He was on the bench across the street from a (hideous) store called "Anderson's Pet Emporium". (John would often wonder why on earth "pet emporium" was the popular choice for pet store names. For now though, he simply shrugged mentally and went back to feeling sorry for himself and the fly that committed suicide in his tea cup.) Mike Stamford had cornered John on his usual "I'm Angry At the World" walk. Despite what John thought was a very convincing "fuck-off" face, Stamford smiled and chatted with John about things he couldn't care less about. John eventually mentioned that he was lonely and Stamford smiled and suggested he get a pet from Anderson's Pet Emporium. So here he was, and John was not impressed.
The pet store was quite gaudy with flashing signs in all colors of the rainbow (and a large amount of gold) declaring that they had the best and rarest pets in the world and multiple others declaring equally unimportant (and untrue) things. John was a bit afraid that the grimy and badly kept storefront behind all of the flashing lights would collapse, but John trusted Stamford. So he took a deep breath and ducked inside.
A bell clanged somewhere in the back of the store as John pushed open the door and slipped inside. The store was a mess. Cages were piled everywhere and in the back, lights from the aquatic section glowed a faint blue. A man a little taller than John poked his greasy head out of a darkened room before ducking inside again. John heard a zipper zip and some hushed whispering before the man, presumably Anderson, exited and slicked a hand over his hair as he extended the other in welcome. "Welcome to Anderson's Pet Emporium, I'm Anderson. Can I help you?"
Anderson's oily smile faltered a bit at John's unimpressed stare. In an attempt to win a place in John's good graces (wallet), Anderson led John around the store for the next hour as John rejected one after another and watched Anderson with grim amusement as he grew more and more desperate. Eventually they reached the back of the store, where there was a tank with a single fish (closer inspection revealed it to be a tuna) wearing a blue scarf. "He won't let anyone take it off. He nearly ripped of my hand when I tried." Anderson replied at John's inquiring gaze. John turned back to the tuna, which gave a scarily toothy grin for a fish with barely visible teeth. "What's his name?"
"Sherlock."
"I'll take him."
— 1 YEAR LATER —
"This phone call - it's er… it's my note. It's what people do, don't they - leave a note?"
Sherlock is lying with his tail flopped over the side of the rooftop of St. Bart's hospital and his mouth pressed to his phone. John's tears are streaking down his face as he gazes up from the pavement.
Moments flicker in front of John's eyes, their relationship since the moment John brought Sherlock home. Scenes of Sherlock using his mouth to grasp slides and eye droppers as he experimented on the sushi and body parts John brought home from the morgue. Cuddling on the couch with Sherlock, repeatedly dampening his scales so that they wouldn't flake, Sherlock demeaning human stupidity. (It was a nice surprise to discover that his aquatic flatmate spoke the Queen's English, less so that he used it to insult almost every aspect of human life, never John's, though. John was the exception.) John had come to even, dare he say it, fall in love with the ridiculous tuna.
And now he was watching Sherlock inch closer and closer to the edge of a rooftop. "Leave a note when?"
"Goodbye, John."
John shakes his head. "No, you can't leave me you ridiculous fish!" He speaks over the snarl of "I'm not just any fish, I'm a tuna" coming from Sherlock. "I love you. I loved you since the moment you spoke to me first and asked where I kept the corpses in my house. I can't let you die now. You are my light and my love. Please."
Too late, he heard a scraping sound and the consulting tuna detective fell over the edge of the building and flailed a bit before hitting the ground with a sickening smack. John rushed over to the tuna and petted him frantically as the fish flopped weakly. He coughed weakly and looked up at John before whispering weakly, "I love you too, you ridiculous human."
John gasped at the admission and sealed their lips in a kiss. Sherlock tasted fishy, but John didn't blame him.
As the paramedics arrived at the scene, John grasped tightly to the body, sobbing salty tears. "Sir, I'm going to need you to let go of the fish."
After Sherlock was taken away John sat sobbing on the pavement, hands stained with his true love's blood.