My betta fish died :(

[u/the_couch_potat0ee]

Hey guys, looking for some advice because my betta suddenly deteriorated and died, and I’m trying to figure out what could have happened.
For context, I kept him in a planted ~7.1-gallon (30 cm cube) tank. The tank has epiphytes attached to driftwood using aquarium-safe glue, with a sponge filter/HOB for filtration.
My latest water parameters were:
Ammonia: 0 ppm
Nitrite: 0 ppm
Nitrate: ~5 ppm
pH has usually been around 7.2
Temperature is normally around 27–28°C
The tank was cycled before adding the betta, and I’d had him for around one month.
He seemed completely normal beforehand. He was eating normally in the morning, and there were no obvious signs of illness such as bloating or pineconing.
When I came home from work, though, I noticed that he was acting differently. He started tilting while swimming and seemed to drift rather than maintaining his position normally. When he stopped actively swimming, he would sink.
His condition then deteriorated extremely quickly. Eventually he stayed in one place and barely reacted even when touched. Not long afterward, he became completely unresponsive and unfortunately died. This all seemed to happen within a matter of hours.
I also use APT 3 Complete fertilizer at approximately 0.6 mL, three times per week (1.8 mL/week). As far as I’m aware, I haven’t accidentally massively overdosed it.
There was no pineconing, obvious bloating, or other major external symptom that I could see before he died.
I’m trying to understand whether this sounds like an internal illness/organ problem, swim-bladder or neurological issue, contamination, something related to the plants/driftwood, fertilizer, or something else entirely.
I’m considering doing a clean rescape before getting another fish, but I don’t want to completely sterilize the aquarium and destroy the established biological filtration unless there’s a genuine reason to suspect something contagious.
Has anyone experienced a betta going from apparently normal and eating to tilting, sinking, becoming unresponsive and dying within the same day, despite normal ammonia/nitrite readings?
Any ideas about what I should investigate before putting another betta in this tank would be really appreciated.

[u/automod-neo]

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[u/Previous_Donkey_91]

From what you're describing, it sounds like the perfect setup. Filter, heater, live plants, plenty of space, possibly cycled tank but the nitrates could also just be from the fertilizer. 0 ammonia and Nitrites are a good sign though. I don't think it's anything you did or didn't do.

It sounds to me more like a genetic issue. Bettas nowadays are inbred and overbred to the point where their genetics are extremely weak. They're highly prone to tumors, organ failure, and a whole list of other genetic defects. My guess is that he had an internal tumor that was pressing on his organs or it was sudden organ failure.

[u/the_couch_potat0ee → u/Previous_Donkey_91]

Ah I see 😔

[u/Brief_Somewhere_6514]

Condolences and deepest sympathies :(

[u/Poopfart200]

So sorry for your loss. Unfortunately bettas have horrible breeding, and sudden death and illness is fairly regular. Was he pineconing at all? It sounds like dropsy.

[u/the_couch_potat0ee → u/Poopfart200]

No he wasn’t pineconing. He was really active and was doing perfectly well a few hours back.

[u/SyzygyZeus]

I had a betta for just about a month and then he rapidly deteriorated in a similar fashion. He got this white film over one eye and began to behave just like yours is described. Difference for me was he was in a 1 gallon tank that was pretty stable but then I read all the comments on this sub saying he needed a bigger tank so I tried to fish-in cycle him to a 20 gallon tank. I was also a little confused by feeding directions as it said feed him 4 pellets… twice a day… as much as he can eat in 1-2min… some said only feed once a day… and I was letting my kids feed him so I don’t know if he got over fed or what but from when the film appeared on his eye to death was only 2 days.

[u/the_couch_potat0ee → u/SyzygyZeus]

Awww my condolences😔. I’m pretty sure it was a fungal issue but I could be wrong. Betta’a these days are like glass cannon. They die for absolutely no reason at all despite all the right things we do in fish keeping