Stumped About Cory Die Offs

[NeonBlueLeon]

Hello folks,
 
Last week, I picked up a batch of six skunk cories for my tank.  Since then, I've had two die-offs, one was two days after introducing them to the tank, and the other was found this morning.
 
Let me cover the basics first:
Tank size: 20 gallon high (75 liters) 24"x12"x16" (60cm x 30cm x 40cm)
Tankmates: 7 rummynose tetras, 1 bolivian ram, 1 creamsicle mollie (juvenile 2cm)
 
Ammonia: 0 ppm, checked twice daily since addition of cories.
Nitrite: 0 ppm, checked twice daily since addition of cories.
Nitrate: ~20 ppm, checked once daily since addition of cories.
The tank has been established for about 9 months now.  I do weekly ~50% water changes, and two or three 10% water changes throughout the week.  Temperature is always matched, and I treat the water with Prime before putting in the tank.
 
The cories were drip acclimated over the course of 90 minutes.  They are the newest members of the tank.
The two cory's carcasses show no signs of discoloration, disfigurement, or injury.  Only one cory had an injured barbel, and that existed from the LFS.  He is still alive.  There is no one picking on them, as far as I have seen.  I watch my tanks obsessively, especially with new fish.
 
The tank is moderately planted.  There are lots of crypts that make something like a canopy about 4-5 cm off my gravel bed.  The cories love to skirt around under that canopy, and usually don't come out.  They usually only come out to take a breath at the surface.  They are more active at night when the moonlight is on.  I also have a piece of driftwood in the tank.
 
Eating has been an issue, I think.  On day two of their introduction to my tank, I put a Hikari Sinking Wafer where they usually hang out.  They all tried to eat a bit, but then the other fish came and took over.  I'm not sure how much they actually ate.  Feedings since then have pretty much gone like that.  I have flakes and small pellets put out for the tetras, ram, and molly.  As they are distracted, I drop a sinking wafer right where they cories are.  They (try) to eat it, until the top feeders finish their food, come down and take over the wafer.
 
Anyway, thanks for reading so far.  What in the world is causing the die-off?  If I left any other important info out, let me know.

 

[mark4785]

The Hikari sinking pellet might be your issue. Any hard pellet that doesn't break up easily is usually ignored by Corydoras as they rely on gulping up small and soft particles from the substrate; they can't bite into or scoop up hard pellets like you'd normally expect from dwarf cichlids, neon tetras, mollies etc.
 
My 4 corydoras love to eat the small particles that drop off one Tetra Tabimin pellet each day; I occasionally swap this for the vegetable variation of the product so that they get a balanced diet.
 
They will completely ignore pellets that don't break up easily until the point that they break up and disperse into smaller segments. If, unlike in my tank, your other fish are eating these small segments before your Cory's can get to them, it is likely they be very hungry / starving.
 
The only time I'd recommend feeding your Cory's Hikari pellets or any other robust pellet is if they nocturnally forage for food while the other's are not looking for the same food source.

 

[Lillefishy]

It could be a feeding problem as corys need a high level of protein in their diet such as bloodworm, brine shrimp etc plus a good catfish pellet. They may not have been getting enough to eat especially as they were new introductions to the tank.
You are doing a lot of water changes too, that could also have a bearing on why they died, one change a week would be enough unless you are very overstocked.

 

[Meeresstille]

For the feeding you could try pre-soaking the wafers before dropping them into the tank, maybe try to feed their tankmates in small bursts keeping them interested at the surface instead of them gobbling the food and then heading towards the cories' food. Or maybe using some sort of divider (plastic canvas or plexi glas?) during feeding to keep them separated. 

   

 

[GetItSahn]

For the feeding you could try pre-soaking the wafers before dropping them into the tank, maybe try to feed their tankmates in small bursts keeping them interested at the surface instead of them gobbling the food and then heading towards the cories' food. Or maybe using some sort of divider (plastic canvas or plexi glas?) during feeding to keep them separated. 

   

+1 presoaking works wonders for my peppered corys

 

[Sophie]

Could it be possible they have some type of internal parasite already with them from the shop? Try and check poop colour and see if they flick against the substrate or decor. That'd be my guess of something that's not obvious. It's an idea - look for bloating/anorexia too.
Corys ideally need sand rather than gravel though - just sayin'.
Stumped other than that.
I wouldn't of thought they would've starved that quickly.. They would still be able to pick scraps of flake up etc just enough to tide them over temporarily. Wack some bloodworm in there.

 

[NeonBlueLeon]

Thanks everyone.
 
I am presoaking some wafer food now, and I'll be using a baster to put it right in front of them.  It would surprise me if the most recent one died from starvation, but I haven't seen them actually eat since I got 'em.
 
Someone correct me if I'm wrong, but I don't think there is such thing as "too many water changes."  My water changes are always matched in temperature.
 

Could it be possible they have some type of internal parasite already with them from the shop? Try and check poop colour and see if they flick against the substrate or decor. That'd be my guess of something that's not obvious. It's an idea - look for bloating/anorexia too.
Corys ideally need sand rather than gravel though - just sayin'.
Stumped other than that.
I wouldn't of thought they would've starved that quickly.. They would still be able to pick scraps of flake up etc just enough to tide them over temporarily. Wack some bloodworm in there.

I'm really really contemplating going through the trouble of switching to a sand bottom.  Now that I have cories, it's even more appropriate.
As rare as seeing cories poop is, I did see one poop, and it looked fine.  But that was just one cory, so who knows.  Again, the most obvious thing I can see is the cories not eating.  We'll see how the pre-soaked wafer goes.  Tomorrow I'll try brine shrimp.  Bloodworms are illegal where I live!

 

[Shaddex]

Sorry for your losses, but what is your tank temperature at ?
Corydoras arcuatus (Skunks) prefer 22-26 Centigrade so if you are above/below this, that could be your issue.

 

[NeonBlueLeon]

Sorry for your losses, but what is your tank temperature at ? Corydoras arcuatus (Skunks) prefer 22-26 Centigrade so if you are above/below this, that could be your issue.

AH!!  Okay I think you just nailed it.  The month has been very warm, and my apartment doesn't have air conditioning.  Some days the temperature of the tank hits 28 C!

 

[Meeresstille]

I've had peppered cories in my tank that was going through ich heat-salt treatment, they just swim to the surface more often for air and were able to handle the increased temperature just fine. So I doubt that 28 Celsius is enough to cause this mass death of cories. 

 

[Lillefishy]

Short term high temps wouldn't cause them to die suddenly, some of my tanks with cory hit 30c this summer without any losses.
The water changing I referred to wasn't meaning the water would be bad, just that by disturbing the tank so often can cause stress & corys can be their own worst enemy when they're stressed & yours had only been in the tank a week so the disturbance could have been a contributing factor in their death.
Other ideas would be they were carrying an illness when you got them & that was why they died.

 

[mark4785]

 

Sorry for your losses, but what is your tank temperature at ? Corydoras arcuatus (Skunks) prefer 22-26 Centigrade so if you are above/below this, that could be your issue.

AH!!  Okay I think you just nailed it.  The month has been very warm, and my apartment doesn't have air conditioning.  Some days the temperature of the tank hits 28 C!
 

 
I've only ever known a temperature of 28 degrees C or above make Corydoras act more fatigued. It hasn't put them off their food and it's certainly never killed them.
 
If, however, you reached a water temperature of 28 degrees C with little to no water surface movement, low oxygen levels could have stressed/killed them.

 

[NeonBlueLeon]

I've only ever known a temperature of 28 degrees C or above make Corydoras act more fatigued. It hasn't put them off their food and it's certainly never killed them.
 
If, however, you reached a water temperature of 28 degrees C with little to no water surface movement, low oxygen levels could have stressed/killed them.

I've got a bubbler and a HOB filter making a bunch of surface disturbance.  The tank is also moderately planted.  I don't think it's a lack of oxygen.
 
I hope it wasn't an illness they carried from the LFS.  Generally, the LFS I buy fish from are healthy, and most of them survive the acclimation process.

 

[NeonBlueLeon]

Haven't had a fatality for a bit.  Hopefully this continues, all fingers crossed!
 
Feeding today went a little better.  I put the usual food for the top-feeders and then used a baster to push some flakes down at the cories.  I saw most of them take a few bites.  Then the top-feeders came down and took over the food again.
 
I've decided to get slightly larger pellets, large enough not to fall through the gravel but still bite sized, for the cories.  Hopefully they will learn to gobble them down before the top feeders come.  My top feeders were clearly overfed today.  Gonna fast them tomorrow, and maybe two days.  Hopefully the cories can survive a two day fast.

 

[HAMBONE]

 
Someone correct me if I'm wrong, but I don't think there is such thing as "too many water changes."  My water changes are always matched in temperature.

yes, you can do too many water changes..