u/LOSNA17LL

Hi everyone,
(Sorry if the writing is messy, I'm still really sad and shocked)
Just learned almost all my fish just died (I'm a student, I don't live at home, so my family takes care of the aquariums for me), and we have no idea why

It's a 120L freshwater aquarium, with a filter, a light and a CO2 dispenser

There were plants (I don't remember all of them, I don't have the labels here with me), including: Anubias, Alternanthera and Echinodorus

And the fish (and shrimps):
3 Ancistrus sp.
2 Corydoras panda
5-10 Rasbora heteromorpha
~30 Neon tetra
~20 Shrimps, mainly japonica

Out of all of them, only the corydoras, 1 neon and 5 shrimps have survived, with no visible symptom

All of the other ones are dead within the last 2 days (2 days ago they were fine, and now they're dead), the bodies (won't show images for sensitive people) all look fine, nothing odd (except that they're dead)

- The last time water was changed was about 2 weeks ago, same time the CO2 dispenser was refilled
- The last fish to come were a batch of neons in February, nothing since
- All the plants are here since last summer (when this aquarium was started)
- We've had an algae issue (likely was Audouinella) for a long time, but it was under control, and it's apparently harmless to fish
- An algae treatment was stopped ~4 weeks ago
- There was no heater, the aquarium was in the living-room at room temperature
- The water test (pH/NO2/NO3/KH/GH/Chlorine/CO2) returns normal values (the GH is kinda high, but we live in the mountains, the water is naturally high in calcium)
- The food hasn't changed recently

So this thing (or these things? we actually don't know if it's only 1 cause) simultaneously killed nearly all our fish, from various species (except some individuals), in less than 2 days, leaving no sign of affliction on the dead bodies nor on the survivors, and there is no recent change in the aquarium

Any idea what it may be?

For now, 75L were changed with fresh new water, all the plants have been cut to a few centimeters, the "lawn" (2 species that I don't remember the name that make sort of a carpet) has been removed
And a bottle of the aquarium's water has been kept to be brought to the pet store for analysis (or so they can say who to bring it to)