Coridora dying 1 by 1 after newcomer introduced...

[CosmicMuffin]

Tank size: 13gal
tank age: 2 years+
pH: 7.0-7.5 ~
ammonia: 0
nitrite: 0
nitrate: normally ~10mg, after 2 weeks without water change - climbs up to 50mg~
kH: n/a (water hard with high chalk content = tons of residues)
gH: n/a (water hard with high chalk content = tons of residues)
tank temp: 24c-26c (75.2F - 79F), currently I keep it at 79F during treatment attempt.

Tap water (not chlorinated, its hard water with ph around 7.5, medium-high carbonate content)
External filter, bacteria established long time ago. Never had a single ammonia spike.
Filter amount is overkill for this size of a tank. (filter designed for 80gal)
I vacuum and refresh water weekly. (30%)
I feed only good quality food which I buy from local breeder.

Hi everyone,

So I basically found 1 long fin bronze cory, took him and put straight into tank...
After 3+ days he was gone.
After 7+ days 1 C. Paleatus died out of nowhere (2+ years old)
After 10+ days 1 C. Sterbai died (2 y old)
After 14+ days my biggest C. Paleatus female died ;( (I still can't accept the fact my Big Momma is gone)

Then I went to shop bought some medication based on metronidazole with some other additives which are not disclaimed (Protosol)
Bough new heater, new termometers (my one was a piece of **** stuck and not actually showing any temperature, same with heater, which was set into 26C, but actual water temperature was 21-20C)
Crazy how I missed this...

Now I treating them, every 48hours I change 30-40% water and add another dose of medication, while totally vacuuming sections of tank to eliminate all the dead pockets and clean from anaerobic bacteria.

Today I found one Albino Paleatus dead..

And the most infuriating thing is that they all die with different symptoms.

My Big Momma swollen completely in few hours, just in the morning she was all happy, in the evening she was double in size, then went upside down and in the morning - gone. (looking like dropsy)
That newcomer died and sinked to the bottom, no swelling or smth.
Paleatus Albino started sinking to the bottom like a rock, in the morning he died, no swelling signs, NO bruises, open wounds, fungal or any other symptoms. Just dropped dead.

Now my 3 Sterbai is a mystery.
2 of them are completely fine, but 1 female is pale and losing color pigment, currently she looks like albino. (attached photos)

While my 2 bronze cories are doing totally fine, female is happy and chonky, jumping around tank like nothing would happen.

Meanwhile 2 sterbai showing eratic behaviour, tripping around the glass and pale female - just calmy sit in one place. She eats.

And kindergarten of glo-tetras are completely fine, they just chilling / fighting as usually.

Maybe someone can guide me, what the hell did I brought into the tank ?
I blame myself every minute for skipping quarantine of new fish...

How to stop this madness???

Would be more than grateful for any help.

 

[GaryE25 23]

It's a quarantine tale. Most of us have been there once, or more. Is it a virus? A bacteria?

You'll never know and because of the lack of knowledge, you'll probably not be able to treat it. It has to burn itself out. I wish the response were better, but that is an unidentifiable pathogen. If it's a virus it's one thing, or a gram positive or negative bacterium - you can't win. I would raise the partial water change routine from once a week to 3 or more times til it stabilizes. Will that help? Maybe.

The cory group hide it well because of their armour hiding internal infection symptoms. That's probably why it presents differently with different victims. These things are discouraging, and for what's worth, I get that.

Here is an issue for the survivors though. paleatus and aenea like those 21-22 temperatures you had. sterbai likes the 25 you have now. They come from different regions and different temperatures.

 

[CosmicMuffin]

My theory about this would be I had dead-zone pockets, which I saw constantly when vacuuming..
Also some Valisneria had root rot, so I took bigger fraction pebbles and pushed around roots to give it more aeration, since then things improved, but anaerobic bacteria was present on long-term in tank..
And my guess I brought in some virus which started heating up the immune system and secondary infection of anaerobic bacteria took over..
Because Corydoras loves digging into gravel, especially the biggest one, she was like a hammer-drill digging inside the substrate and reaching pockets of gasses...

Guess my choice to mix different size of pebbles backfired hard

 

[GaryE25 23]

If this is the case (when I've seen the losses you describe it's been straight up disease brought from the store, with no quarantine) you can solve it by buying sand, rinsing it and pouring it in. It'll fill the gaps between pebbles, and give the corys (actually Osteogaster aenea and Hoplisoma sterbai and Hoplisoma paleatum) their natural sand sifting possibilities.

 

[CosmicMuffin]

What is the strangest thing, after my pale Sterbai passed away, since that day no other fish died.
Today I will flush the tank again and put the carbon filters to remove medication.

Also Glo-tetras / Kribensis cichlid seems to be completely unbothered by the epidemic.. Which is strange, because only bottom dwellers got affected by this pathogen..

I was thinking about sand since the beginning.. But I have limited width fish tank and its very heavily planted one.
+ other hobbyist warned me that thick sand layer will completely trap the oxygen exchange and create even more problems with anaerobic pockets...

In future I had plan to get much thicker fish tank (like 45cm) so I would have space to create sandy center (with 3d printed barriers to separate sand mixing with substrate) and correct substrate for plants to thrive (not river pebbles, which are very basic/bugdet choice).

But thanks for help anyway.

 

[GaryE25 23]

I expect a group specific virus introduced with unquarantined fish ran its course and has burned out. The Cory group are known to have problems that don't affect the entire tank.

If you are concerned about anaerobic pockets in the sand, get a stick and periodically poke the substrate. I use sand or fine gravel and in 60 years have never had a problem with pockets of decomposition.