It happened over the last months, in several different tanks, during and after quarantine, with various (but adapted to the species) water parameters.
To break this down:
Tap Water pH: around 8
Depending on fish: Mix of tap and RO or rain water, or full tap (treated with JBL conditioner)
No Ammonia.
No Nitrite.
Little Nitrate in planted tanks. Heavily planted tanks when it happened there. Little to no nitrate in quarantine tanks.
Water on the harder side in general, with higher Gh/Kh than the norm in tap water. Tempered with measured amount of RO/Rain for half of my tanks (for species that do not thrive in hard alkaline water).
All the tanks that are concerned were heated to the appropriate temperature for the fish, again, this is not an isolated incident, but an ongoing mystery.
I feed live food (snails, daphnia, brine, baby brine), frozen food, and quality dry food, appropriate to the fish. The problem seems unrelated to what I feed since it happened to fish fed entirely different foods.
Now let's get to the sad part:
First female betta died suddenly, 1,5 years old fish. After a water change. 1 day later lethargy, no appetite. 2 days later: dead. No visible symptom on the fish. No other fish in contact had any problem.
Female pea puffer, 1,5 years old fish. Died suddenly. After a water change. 1 day later lethargy, hiding in the riccia, no appetite. 2 days later: dead. No visible symptom on the fish. Male puffer in the breeding tank: no problem. Still alive and well today. Attributed at the time to breeding stress that i might not have observed, it seemed to work pretty smoothly.
I then bought a female betta. Ate fine in the quarantine tank. Day 2, loss of appetite, lethargy. Day 3: dead. No visible symptom on the fish.
Later the same month I did it again, bought a female betta, and a couple of gold Apistogramma Cacutoides. Female betta in quarantine tank (with new water, partially treated tap), couple of cacutoides in an established heavily planted tank full of wild neocaradinas and no other fish. Female betta died in 3 days, the same way as the precedent one, different quarantine tank, no common element or equipment besides... the tap water.
This week, I do a partial water change (30%) in the Apisto tank and in another heavily planted guppy tank.
Two days later, the female Apisto doesn't eat the live BBS and is lethargic. The day after, I found her dead. No visible symptom on the fish. I was extremely sad as I truly loved this couple, the male looked sad, searching for his mate, but seems in good health.
In the guppy tank, same day, I find a female guppy dead (1 year old female, born with a defect - missing one eye, never bred in one year, but still very healthy before the water change).
Guppies seem to be mostly safe from the mystery death. Male bettas are healthy. Male puffers are healthy. Pygmy corydoras are healthy (but they didn't get much tap water the last couple months).
We got a handful of gold white cloud minnows that were sick in the mean time but they were in a dedicated tank, and treatment (furanol) was effective. The mysterious deaths started before this acquisition and there has been no contact between this tank and others, mistake could have happened though so I'm not ruling anything out (it could be random deaths and later cross contamination with the white clouds, but I don't believe it, white clouds did not suffer sudden death, they showed clear signs of bacterial infection from the start and all survived the treatment).
I am a little paranoid about my tap water now (french city water), as it seems to be related to water changes involving tap.
Why only females though? And what could cause such sudden deaths?
I maintain about a dozen tanks of different sizes, breed fish (puffers, corys, guppies...) with relative success. These sudden deaths are puzzling me and I am worried about the rest of my fish, and even more about my female fish (I still have several female puffers, and a bunch of female pygmy cories, khuli, rasboras, goldfish...).
I know most of the Coop audience is from the U.S. so European (French) Tap water is far from being the same. Still, maybe one of you could have an idea, or know someone who faced something similar.
Thanks a lot for your help.