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Corydoras Habrosus dying by the week

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I'm hoping that someone on here can help me out. I have a 40 gallon breeder with Corydoras Habrosus, Glowlight danio (choprae) and a few hillstream loaches. The substrate that I am using is black diamond blasting sand. I started out with 21 Corys in the summer. I had them in a tub outside on my back deck and they did great all summer. not one died even when the water temperature was over 90 degrees some days. Every week I am finding one dead in my tank. All of my danios are doing great so I don't understand what the issue is. I perform weekly water changes and feed the fish a very healthy diet. Sometimes I will catch a Cory swimming upside down or on its side then later finding it dead a few days later. I will provide photos below. The photo of the fish on its side was taken this morning, it is alive, it tends to turn on its its side and upside down. The other photos are just showing the other Corys and their general health.
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Discussion Starter · #4 ·
Also any heater
yes, heater is always set to 77°F

Provide your water parameters, filter setup, how long it’s been running, etc. Standard stuff.


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PH 7.5
Ammonia 0ppm
Nitrite 0ppm
Nitrate between 0ppm and 5ppm
KH 2 (25ppm)
GH 3 (50ppm)
Temp 77°F

I live in New York, not sure if the tap water here has anything to do with it. I have 3 filters in the tank, a large sponge filter that has a power head connected to the top, a medium sized sponge filter on the glass, and a large box filter that has purigen in it. I was using the purigen to clear the water because my drift wood was leaching a ton of tannins in the beginning of my tanks life. The tank has been set up since september 20th. Unfortunately I just got home from work to find one dead cory and another one laying on its side but it ended up turning over and swimming away. i have a photo of the dead one, looks like it has some blood on the bottom/side
 

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If you're treating your tap water and you haven't changed anything else about the tank, a few questions:

Were the Cories fully grown when you got them?

What do you feed them? How much? How frequently?

How much water do you change each week? Do you treat it with something like Prime before you add it to the tank?

What's your fertilizer situation?

They generally need higher kH and gH. While doubtful that's the culprit, it could still cause problems if those values or the chemical makeup of TDS has recently changed. How are you testing kH and gH? Liquid? Strips?

Do you have the means to test if there is stray electrical current or anything in the tank? Is it possible something in your equipment has been exposed that could occasionally zap your tank? May want to turn everything off, let it cool down and do a bit of an inspection.

If nothing is out of the ordinary, my gut goes to Black Diamond. It's a coal by-product and often has an oily film even after rinsing. While usually okay, it's still not always completely inert as some would like to believe. Your water is soft and acidic, so it could theoretically be causing some kind of breakdown of the coal slag, potentially releasing any number of chemicals and metals. Though, I'm not sure there'd be a way to tell unless you shipped some water samples off to be tested - usually about $40 a pop.
 

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Discussion Starter · #7 ·
If you're treating your tap water and you haven't changed anything else about the tank, a few questions:

Were the Cories fully grown when you got them?

What do you feed them? How much? How frequently?

How much water do you change each week? Do you treat it with something like Prime before you add it to the tank?

What's your fertilizer situation?

They generally need higher kH and gH. While doubtful that's the culprit, it could still cause problems if those values or the chemical makeup of TDS has recently changed. How are you testing kH and gH? Liquid? Strips?

Do you have the means to test if there is stray electrical current or anything in the tank? Is it possible something in your equipment has been exposed that could occasionally zap your tank? May want to turn everything off, let it cool down and do a bit of an inspection.

If nothing is out of the ordinary, my gut goes to Black Diamond. It's a coal by-product and often has an oily film even after rinsing. While usually okay, it's still not always completely inert as some would like to believe. Your water is soft and acidic, so it could theoretically be causing some kind of breakdown of the coal slag, potentially releasing any number of chemicals and metals. Though, I'm not sure there'd be a way to tell unless you shipped some water samples off to be tested - usually about $40 a pop.
The corys were pretty close to full grown when I got them back in July. I grew them out to adults pretty quickly with the help of lots of live food coming from my fish tub outside. I currently feed them sinking multi worm pellets from Ken’s Fish. they also pick up flakes and any frozen food that sinks to the bottom. My danios do tend to steal the pellets from the corys and move them to the other sides of the tank (the pellets are very light). I’m feeding about 5-6 days a week and I feel like I feed the right amount, some times i’ll throw a pellet in the tank and it will last 2 hours before the fish finish it.
I change 50% water every week, and I treat the tap water with prime.
Under my BDBS I have regular organic potting soil.
I am testing hardness with the API liquid.
I have had my hands in the tank multiple times and never got zapped, but that’s still a very interesting point that has me thinking, might have to take a look at the wire on my heater.

Are you treating the tap water for chlorine/chloramine with your water changes?
yes i am using prime during my weekly 50% water change
 

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Your water is soft and acidic, so it could theoretically be causing some kind of breakdown of the coal slag, potentially releasing any number of chemicals and metals.
The pH is 7.5, so not acidic.

I have the same parameters as OP. Soft, but basic water (pH 7.6, KH 3, GH 4) I also tried to keep Cory H. with the same results. I also had them with Glowlight danios in a mature planted tank. I lost about one per week until they were gone. All other indicators in the tank were good. The corys never showed distress. The Danios never bothered them. They acted normal right up until they died.

If that happened again, I'd put them in a QT right away and try to save them.
 

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The pH is 7.5, so not acidic.

I have the same parameters as OP. Soft, but basic water (pH 7.6, KH 3, GH 4) I also tried to keep Cory H. with the same results. I also had them with Glowlight danios in a mature planted tank. I lost about one per week until they were gone. All other indicators in the tank were good. The corys never showed distress. The Danios never bothered them. They acted normal right up until they died.

If that happened again, I'd put them in a QT right away and try to save them.
They're using an API test kit, which isn't what I would consider reliable, and have a low kH. Low kH = likely lower pH than they think they have in such soft water. Soft on both the kH and gH front and likely softer than the API kit reveals, as those reagents expire quickly. Hence my remarks about their water being soft and acidic and concerns about more ideal parameters for the species.

But my gut still goes to the coal slag in this instance. It's just not as safe as some would like to believe.
 

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Discussion Starter · #10 ·
The pH is 7.5, so not acidic.

I have the same parameters as OP. Soft, but basic water (pH 7.6, KH 3, GH 4) I also tried to keep Cory H. with the same results. I also had them with Glowlight danios in a mature planted tank. I lost about one per week until they were gone. All other indicators in the tank were good. The corys never showed distress. The Danios never bothered them. They acted normal right up until they died.

If that happened again, I'd put them in a QT right away and try to save them.
I think i might’ve solved the issue. The glowlights are very cocky when they eat and they steal the pellets from the corys. i’ve been feeding a lot heavy and i’ve been getting a ton of granulated foods to the bottom and since then i haven’t had any dead corys. it’s only been like 3 or 4 days but so far so good. maybe they were starving to death? it’s hard to tell, they’re small to begin with already lol
 

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Their bellies are a good indicator, should be slightly bulging, never sunken or flat. I feed frozen cubes that slowly fall from the top to bottom and I feed before lights out with the sinking stuff to give the cories a chance. Good food, insect based, is crucial for them, fluval bug bites for example
Several people didnt have luck keeping corydoras on blasting sand, since it is super sharp, so that could be an issue, also API stress coat contains aloe vera, which is poisonous for fish
So just an observation
 

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Their bellies are a good indicator, should be slightly bulging, never sunken or flat. I feed frozen cubes that slowly fall from the top to bottom and I feed before lights out with the sinking stuff to give the cories a chance. Good food, insect based, is crucial for them, fluval bug bites for example
Several people didnt have luck keeping corydoras on blasting sand, since it is super sharp, so that could be an issue, also API stress coat contains aloe vera, which is poisonous for fish
So just an observation
Super sharp? Compared to what?


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