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This is a problem that has plagued my Bronze Cories for many months now, and no one has even the faintest idea what is happening. I don't think it has any environmental cause, but I know it's important to establish the environment these guys are in.

1. Water parameters
a) Temperature range
74-76 F (colder in the winter, warmer in the summer)

b) pH
7.8-8.0

c) GH
8-12, depending on how many days since I added ferts.

d) KH
4-6

e) Conductivity or TDS (if GH and KH aren't provided)
n/a

f) Ammonia, Nitrate, Nitrite levels (Most LFS's will check your water and give a list of readings)
0ppm Ammonia, 0ppm Nitrite, 10-20ppm Nitrate

g) Water change frequency
Weekly, 20%-40% depending on what is needed.

h) "Routine" water treatments (e.g, chlorine/chloramine or ammonia neutralizers; pH or hardness adjustments, anti-stress chemicals, tank-cycling bacterial mixes).
Seachem Prime, Aquarium Co-op EasyGreen

2. Tank set up
a) Size
75 Gallon

b) Substrate
Caribsea Peace River Gravel (They do not have an issue with this gravel, they find food easily and have long, intact barbels)

c) Filtration
Marineland Penguin 200, Cascade 1000 Canister filter, a sponge filter

d) Furnishings
Marimo Moss Balls, Java Fern, Pogostemon Stellatus Octopus, Cryptocoryne Wendtii, several caves and hides.

e) Other tank mates
2 Adult Bronze Corydoras
4 Peppered Corydoras (I need to get more)
15 Cherry Barbs (Irrelevent, as this problem has been going on since before I added them)
Bladder Snails

f) How long has it been set-up?
February 17th, 2019

g) Food used and frequency
Once a day feeding of any combination of Hikari Sinking Wafers, Omega One Catfish Sticks, Omega One Sinking Pellets, and Hikari Micropellets. Once a week I feed thawed frozen Brine Shrimp and Bloodworms, Omega One brand. Occasional treat of blanched veggies for them and the Cherry Barbs.

h) Recent changes in the tank which occurred shortly before the disease/problem appeared (if any; e.g., changes in water source or water treatment, changes in decorations or substrate, replacement or changes of hardware (filters, heaters, etc.), and additions or removals of live plants or live fish)
Has been going on for months, so n/a.

3. Symptoms / Problem description or history
This problem afflicts only the Bronze Corydoras which I raised from a single clutch that I hatched in June 2018. None of the other Cories have developed this. The symptoms are always similar. The Cory becomes lethargic, looses its appetite, and breathes rapidly. They often become dark in color and sometimes, but not always, their eyes cloud over and they loose vision. They never live for more than a few days after the symptoms first present. It only ever effects one Cory at a time, and there's always a period of time between one showing symptoms. I don't believe it's related to water quality or diet, their foods have plenty of Vitamin A and I'm diligent about keeping things clean.

I've speculated that this is some sort of genetic problem. Their parents were chain pet store fish, and though their mom has always been very healthy, their dad was not the most robust, and he passed away without any cause or symptoms not too long ago. The fry did not have a good survival rate, and there were genetic abnormalities noticed in some of them as they got older. (Mainly, some of them developed very long fins, and all of those died from this condition.) They also are slow growers, at nearly 2 years old they are nowhere near adult size. I'm down to 11 from over 30 juveniles that grew big enough to put in with the adults, most of them dying from this issue. I have yet another one who started showing symptoms last night. This Cory was absolutely fine the day before.

4. Action taken (if any)
I have tried quarantining some of them and treating them with antibiotics, because the symptoms show some similarities to a bacterial infection, but they always die within a day or two of starting treatment. It seems like whatever this is, by the time symptoms show they're too far gone to treat.

5. Medications used (if any) / changes in fish observed since treatment began (if any)
At different times for different fish I've used Furan-2 and Amoxicillin in a 10 gallon quarantine tank. The fish always died overnight or the day after treatment.

I'm at a complete loss here, I've asked other places and no one has any idea other than complete shots in the dark. This has been very hard to deal with, and I feel completely helpless. If anyone has any idea what this could be, or knows someone who might, please let me know.

Below are images I've taken over the course of many months, in chronological order. Each one is of a different Cory who had this condition, the last one is currently suffering from this. The first one is one of the long fins I mentioned earlier. I'm linking them because the images are very large.

https://i.imgur.com/ALKc5MA.jpg
https://i.imgur.com/4Smpjb4.jpg
https://i.imgur.com/QlrQvDg.jpg
https://i.imgur.com/Zxs24th.jpg

Those cloudy eyes are weird. There are several different causes of cloudy eyes. Reportedly, low pH can cause it, and also some parasites. Your pH is on the high side. I don't know if high pH can do it too.

Maybe something with the ferts? I don't use fertilizers in any of my tanks, so I'm only speculating. Hopefully someone else can offer advice too.

Good luck,
Eric

bekateen wrote: ↑22 Apr 2020, 04:19 Those cloudy eyes are weird. There are several different causes of cloudy eyes. Reportedly, low pH can cause it, and also some parasites. Your pH is on the high side. I don't know if high pH can do it too.

Maybe something with the ferts? I don't use fertilizers in any of my tanks, so I'm only speculating. Hopefully someone else can offer advice too.

Good luck,
Eric

Hi Eric!

My pH is naturally high, it comes out of the tap at 7.6. These fish have been born and raised in this water, so I would be surprised if that's the problem. I did add some driftwood to the tank the other day, which might lower it a bit.

I don't think it's the fertilizer, it's quite a mild mixture as far as fertilizers go, because they created it to be completely safe for scaleless fish, snails, and inverts. (Because of that, it has no copper.) That's why I bought it, it's a pretty idiot proof, basic fertilizer. It also apparently has a very long shelf life, the guy who made it recently mentioned that the very first bottles produced are still passing the longevity tests at the testing facility. So I don't think mine has expired either.

It's just baffling. The worst part of this has been not knowing. Having no idea what this is, and not being able to find anyone who has seen anything like it.

Not knowing much about Corydoras, but I graduated in Chemistry, I would be very carefull with ferteliser

Regardless the formula, plants need potassium, nitrogen (that is, ammonia, nitrites nitrates) and phosphates. Further they will need some trace minerals, such as copper, selenium iron and manganese. All in very, very small amounts.

These trace minerals will most likely not be the problem, most often they are combined with chelates such as erta, which bind to the metal, reducing both the toxzicity and its tendency to oxydize (quite a problem for iron)

The problem is, however, the nitrogen and phosfates. These are all waste products for fishes - and thus they are, some more, some less, harmfull to fishes.

Most of us keep more, much more fishes in a tank than nature does in a similar amount of water, and the fish will therefore produce excrements. Containing precisely what the plants need.

I've kept forests of Saggittaria on half an inch of sand, which was unwashen once, for 5 years or more, with a lot of fishes.

Any nutrient the soil contained, must have been used after 2 years, but the large amount of fishes - or better put, feeding them, kept nutrients in the water and thus kept the plants growing.

You will not need a fertiliser, but it can, it might, do harm to your fish.

Bas Pels wrote: ↑22 Apr 2020, 19:44 Not knowing much about Corydoras, but I graduated in Chemistry, I would be very carefull with ferteliser

Regardless the formula, plants need potassium, nitrogen (that is, ammonia, nitrites nitrates) and phosphates. Further they will need some trace minerals, such as copper, selenium iron and manganese. All in very, very small amounts.

These trace minerals will most likely not be the problem, most often they are combined with chelates such as erta, which bind to the metal, reducing both the toxzicity and its tendency to oxydize (quite a problem for iron)

The problem is, however, the nitrogen and phosfates. These are all waste products for fishes - and thus they are, some more, some less, harmfull to fishes.

Most of us keep more, much more fishes in a tank than nature does in a similar amount of water, and the fish will therefore produce excrements. Containing precisely what the plants need.

I've kept forests of Saggittaria on half an inch of sand, which was unwashen once, for 5 years or more, with a lot of fishes.

Any nutrient the soil contained, must have been used after 2 years, but the large amount of fishes - or better put, feeding them, kept nutrients in the water and thus kept the plants growing.

You will not need a fertiliser, but it can, it might, do harm to your fish.

I've had fish develop this problem during periods of weeks or months where I didn't fertilize because I was trying to control algae. Many, many people have Cories in tanks that they fertilize, yet this problem, whatever it is, seems to be exceedingly rare.

I can't tell you why this is happening to you but that cloudy eye sure look odd, I have never seen that before. I'm sorry for your loss. During this pandemic I have lost all my Pandas, all my Pygmy's and there is only one trilineatus left. The C. Aeneus, C. Paleatus and B. Splendens are rock solid.

I hate the fact they're dead, but I hate even more that I can't go to the store and get some because the store is closed.

Again I'm sorry for your loss I understand.