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I have been losing 4 C. Duplicareus over the course of about a month. (also 1 L 201) Thats in a tank that has run without incidents for 18 months.

I did however make 2 recent changes, and the cause is probably to be found in one of those. I really need to know which. Kindly read below.

1. Water parameters
a) 25-27C
b) pH 7,4-7,8
c) GH 21+
d) KH 15+
e) Conductivity 330PPM
f) Ammonia 0ppm, Nitrate 0 ppm, Nitrite 0,05-0.1ppm levels (This is the read I get even from rainwater/tap, and tank, its all in that range)
g) 25-30% weekly
h) Have been adding catappa leaves and Tannin aquatics botanicals the whole time.

2. Tank set up
a) 800L
b) Fine Sand
c) Eheim 2180, 3 trays of 4L biohome. Also a Maxpect gyre 250 for increased flow. (slightly underfiltered?)
d) 3 large pieces of driftwood
e) 20 C. Duplicareus (now, 16) 200 Cardinals, 4 L201 (now 3), 4 L091, 4 L075. Some Amano shrimps and Helena snails
f) 18 months
g) I feed every other day, tuesdays, thursdays and saturdays. 3 times during a feeding day, to make sure everybody gets a piece.
h) Recent changes:

1. April 2018: Started mixing in 50% rainwater for my water changes. Filtered manually through foams, and then aerated in a holding tank. I mixed it directly into the tank. 1 pipe of rainwater, 1 pipe of tap. I now know that that is not the way to go. I should mix it before hand. 7 water changes, and 3 new Duplicareus, before I lost the first cories.

2. Beginning May 2018: Added 3 new C. Duplicareus (20 total) I didnt quarantine but over about 1 hour added them to my tank.

Because of the rainwater mix, over the course of 7 weeks my parameters changed to:
TDS 237ppm (from 330), GH 14 (from 21+), KH 10 (from 15+)
This is where I added the 3 new Duplicareus

About the middle of may 2018 I lost 2 Duplicareus within 2 weeks. After that, Starting at the end of may we had a draught, and no rain for weeks. After 2 weeks of waiting, I HAD to do a waterchange and did it with pure tap water. And the parameters shot up to:
TDS 280ppm, Didnt measure the rest cos I'm lazy. But they were probably back up high. Please note however, that I did lose 2 cories before this 100% tap water change.

My rainwater system has failed and I have given it up, until I get a proper RO unit. Readings are now back to TDS 330ppm, GH 21+, KH15+.

The guy who sold the C. Duplicareus to me, who is a reputable aquarist, said they had been with him for a long time with no problem. One of them DID have some kind of tumor though, just left of its mouth. About 1mm in diameter. Orange in color. I realized this after I bought them. But this also made it possible for me to recognize it later, and determine that it was in fact 4 other C. Duplicareus that had died. At least two from my old stock. The tumored one, digs through the sand as happy as ever.

3. Symptoms / Problem description or history
25-30% Water change history last 2 months:
Old Parameters: A pretty stable TDS 320-340ppm
April 9: 50% Rainwater, TDS 303
April 16: 50% Rainwater, TDS 272
April 23: 50% Rainwater, TDS 273
April 30: 50% Rainwater, TDS 249
May 7: 50% Rainwater, TDS 235
May 14: 50% Rainwater, TDS 237
May 27: 100% Tap, TDS 280
June 4: 40% Rainwater, TDS 270
June 11: 100% Tap, TDS 297
June 24: 100% Tap, TDS 330

Dates are not entirely accurate, but the other data is.

4. Action taken (if any)
After I lost my L201 to what appeared to be Finrot, I started a 3 day Esha 2000 treatment. About a week after this, and 3 days after water change (today) I lost another C. Duplicareus. Hence this post!

5. Medications used (if any) / changes in fish observed since treatment began (if any)
Esha 2000. 1 more dead C. Duplicareus, now 4 in total. 3 before the treatment.

Writing all this, makes it seem more obvious that it was my reckless play with water parameters that took out my cories. But I still need another take on it, and decide if it was in fact not the 3 new additions.

Have I taken my poor fish on a roller coaster ride? or was it the 3 new additions?

Forgive me if I have not read - but what do dead fish look like? (Corydoras duplicareus)

I haven't the heart to take a picture of my dead Cories. But It quickly becomes just the shells of a fish. Much like the shelling of shrimps.

I now read in another forum: http://www.plantedtank.net/forums/11-fe ... e-tds.html

That for delicate fish, a change of 10% in TDS can be fatal, as the osmotic pressure changes, and makes it hard for these fish to create the necessary osmotic balance. In essense, when I lower the TDS of more than 10% at any one time, I risk drowning my fish, or the other way around. At worst I have been making an 18% change, and usually more than 10% changes.

I made 50/50% rainwater/tap water changes from the get go (sometimes 100% rainwater, sometimes 100% tap. I should have started by doing 10%/90% instead, and then gone up from there. If this is indeed the cause of my losses, its a very important lesson for me to learn indeed.

I doubt it was the TDS change alone that killed your fish. Duplicareus are not that delicate.
Some breeders of l-numbers do quite drastic changes in TDS to get them to spawn. (Lowering it)

I don't know what exactly caused it. My guess is old age and maybe shock for the other two after the big water change (maybe already weak).

I think you know your water is quite hard and duplicareus is a soft water fish, which is why you're getting an RO unit.
In my experience corydoras kept in hard water are just more prone to health problems.

If I read

But It quickly becomes just the shells of a fish. Much like the shelling of shrimps.

then this sound as if the fish decayes within 24 hours, most likely even faster.

I once had a problem in a tank with a few deseases atthe same time. A fish could swim as if nothing was wrong, and 2 hrs later it could be dead and smell as if dead for 24 hrs.

I lost all the fish in the tank.

Its just, nothing happens for about 18 months. Then I start playing with rainwater (and adding 3 more C. Duplicareus) and suddenly within a few weeks, fish start dying. And what about the strange finrot of my L201? Did they fight? They have massive space and are still immature..

Either these 3 newcomers brought some infections with them (they were in a 300L holding tank stocked full of other corries and smaller cichlids, and/or maybe the water changes stressed them, and thus they started dying. So maybe a mix.

Point is, be more careful i guess. Don't try different things at the same time. Makes it very hard to determine the cause of any ill effects.

I want to add 10 more Duplicareus and a few more L 201. But not before I have this spell of slight misfortune far behind me.