Murder in a Bowl: Why Your Pet Fish Died

I. Mystery

December 26, 2024, a day I remember well. I had just been bought by a human child and moved into his aquarium. A solitary betta fish like me never jived well in new, and especially overcrowded neighborhoods, and the gossip from being the sole private investigator never did me any good either. They say you always remember your first job. In this case, it would also be my last…

*phone rings*

Betta: Yes?

Caller: Is this Detective Betta, P.I.? 

Betta: Yes, what’s the case?

Caller: My roommate, I don’t know why, but he’s not breathing. He was just fine a while ago I–

Betta: Alright, I’m all ears. Where are you?

Caller: The Northwest Corner, near the anubias.

It was a large sand-bottomed tank, dotted with rocks and ornaments, but almost devoid of plants. The wide, green leaves of the anubias made the area easily distinguishable from the gray and off-white surface. A peculiar sticker was pressed against the North wall. It looked like a clock, with every three-hour slice shaded a certain color: yellow, pale teal, powder blue, and azure. Each quarter circle had strange symbols behind it. At its center was a circle that matched the azure of the two-to-five o’clock region. Weird, must be for the tank owner. When I arrived, I was met with a beautiful, graceful discus.

Discus: Thank goodness you came! I was beginning to panic! 

Betta: You’re very welcome Ma’am, but, where is the body?

Discus: Oh sorry– he’s right behind me.

As she swam aside, a rotting stench wafted through the water. There lay the rounded corpse of a gourami. Bloody patches lined the still-shining red-and-blue scales, indicating a recent death. The eye was cloudy, mouth agape. 

Betta: Hmmm, what a bizarre case.

Discus: You’ve never seen anything like this before?

All my life, I had fought to survive. I had seen too many fish bruised up and bitten with chunks of their flesh taken from their resilient bodies. I had even seen the desecrated, half-eaten remains of a large goldfish. But this seemingly benign scene startled me. How can something die from just this?

Betta: …Well, I guess I have. This could be a murder by poisoning perhaps. I’ll just begin my examination.

Discus: Just get it done, please? I have the blood worms.

A rare delicacy in an age of pellets, bloodworms provide both a delicious meal and a nutritious source of protein.

Betta: Thank you.

II. Clues

It had been hours, and my examination led me nowhere. I discovered internal bleeding, but no visible bruising or scale damage, so blunt force trauma can be ruled out. It wasn’t a swim bladder infection either, otherwise the body would be floating near the top. Maybe a parasite? But there weren’t any eggs or white spots either. For crying out loud, what could it be? I needed more clues… the crime scene should give some answers.

I was met with another gruesome scene, made even worse by the fish swimming before me.

Angelfish: Who are you? What’s your problem?

Angelfish, what a nasty bunch. The bosses of the tank control large swaths of territory on the floor, and therefore the incoming pellet drops. Their wars over these are long and brutal. Still, I felt brave enough to challenge this one. He seemed to act more lethargic and less hungry–something his rivals seemed to exploit, which taxed him more in return.

Betta: Nothing… just here to investigate what’s behind you.

Angelfish: In my territory? No. Get out.

Betta: Look, I can see the body behind you, and you know when they rot, it ruins the water nearby. It’s not like there’s circulation anyway in this aquarium. 

The lack of circulation had always stood out to me. In all my previous solo tanks, I had always had a constant flow of water, especially near the walls. Even if it wasn’t strong, I could tell when the water was disturbed by some machine of the humans or by a fellow fish. This new tank only had the latter.

Angelfish: Fine, but don’t stay too long or get in my way.

As I swam closer, I realized that the corpse was of the discus. What was once a multicolored beauty akin to an extravagant fine china plate lay flatly on the ground, pale, lifeless. I had already learned not to grow too attached. Surprisingly, this was not caused by an angelfish attack, even if it was plausible. I looked on hurriedly, scouring the body, noting all the details I could. Bloody patches, hemorrhaging, with the corpse remaining on the ground. Same as before. But there was something different; maybe I hadn’t noticed it before with the gourami. The gills were inflamed, bright red–

Angelfish: Time’s up. Scram.

Betta: But I had just–

Angelfish: Nope, get out.

III. Truth

Back at home, my perplexity only entrenched itself in my thoughts. I had already ruled out infections, given a lack of white spots and ulcers, and fishicide was out of the question. Still, I never had time to fully examine the area given the surprise of and subsequent focus on the discus’ corpse. I was sure that the gourami and discus died the same way; the details were already resurfacing in my mind. I was beginning to lose my cool as a sinking feeling of dread crept into my already-shaken consciousness. A final inspection of the surroundings should yield results – I hoped.

As I made my way back, I spotted the sunken corpse of the angelfish. Before I could respond, the stench characteristic of the previous death scenes punched me straight in the nose. It now pervaded and overwhelmed the whole of the tank. I now began to rush.

There it was, the place where it all began–the North wall, with its anubias, sand, and–wait, the sticker! 

I rushed to the mysterious circular emblem, shimmying my way between it and the wall to get a better reading of the symbols. It was text. My eye darted to the nearest label:

AMMONIA ALERT

What? Isn’t that what our feces dissolves into? Why would the owner be tracking that? I checked the text in front of each of the color and–

My heart sank.

TOXIC

0.5 ppm

It was then I realized what had been killing us all along: the tank itself. All of our feces had been rotting for so long that it was poisoning us, with barely any plants to absorb it and no machines to clean it. I had only survived this long on a ration of bloodworms instead of those manufactured pellets. 

As I nervously stared out through the glass walls, what had once been dread had now transformed into nausea in my head and a searing pain in my gills. The symptoms had surfaced. 

I knew the end was inevitable.

TLDR:

When your pet fish dies due to seemingly nothing, it’s most probably ammonia poisoning. In an improperly maintained tank without cycling (water replenishes and filters), feces can rot into ammonia, which poisons fish.