Corydoras sick and dying

[NCfish]

My Corydoras are displaying some strange symptoms and I could use some advice.  I'll give as much info as I can and then give a few guesses as to what's going on.  Warning, this is a long read.  

Some important context.  My city tap water changed on the 1st of the month as part of their annual system flushing and this was my first w/c since that change.  Per the city website; "normally uses chloramines, a combination of chlorine and ammonia, for the disinfection process. The State of North Carolina encourages all water systems that add ammonia to cleanse their system annually by switching to chlorine only and flush the system"

I have 2 (1 passed tonight 3/11) Corydoras paleatus females around 6 years old in a 20 gallon tank with 15 cherry barbs of varying size (None of the barbs are showing any symptoms or strange behavior.). Did a 30-40% water change Thursday night and by Friday midday 2 of them started acting shy, not searching for food, rapid gill movement and trouble orienting themselves when swimming progressing to twirling in circles.  They are darting to gulp surface air a normal amount (as most Corys do)  They could have been displaying some of these symptoms earlier and I didn't notice, but not much earlier.  Max 48 hours.  The one female is not displaying any symptoms and is acting normal.  She has always been the largest and healthiest.  All of these symptoms have gotten worse friday night and saturday. I added an airstone to offgas any residual Chlorine on Thursday night after water change. One female passed and the other sick one is looking on deaths door lying on her side on the bottom w/ continuous gill movement.  Heavily considering euthanizing her.  I have good video of symptoms but don't know how to upload a video.

They have no cosmetic signs of disease whatsoever.  Very good coloration, plump, healthy barbels, no streaking or discoloration of fins.  

On Friday night I reached out to a catfish keeper in my Aquarium club.  She suggested I do a large water change with storebought spring water.  I did a 60% water change with remineralized distilled water because that's all I could find.  2x dosed prime with this waterchange. Added several square inches of Polyfilter. No improvement.

This tank is slightly overstocked but I've found the water lettuce under high light scrubs the water very well and I remove several handfuls of biomass every few weeks. Hence why an ammonia spike seems unlikely. Nitrates never get above 25.

Tank parameters: 20 gallon long tank 3 years old with sponge filter.  Lots of water lettuce and water sprite.  1/4 inch pool filter sand and some leaf litter (magnolia and oak) pH 6  GH 3-4 KH 0-1  Ammonia 0  Nitrate ~25 Nitrite 0  Temp 71F

Tap water parameters as of 3/11  pH API says 7.6 or higher.  Co-op test strips read 6.4 so not sure what that's about. GH 3-4 KH 2-3  Chlorine at least 1.5 ppm

My current theories are;  1. There somehow was residual chlorine in the water even after aerating 3/4 of the water for 24hours and double dosing prime.  A few of the gallons I added during the water change were not aerated overnight but still dosed with Prime.  I've read Corydoras can be especially sensitive to chlorine. 2. A slight ammonia spike from water lettuce root dieoff (which seems to come in cycles. They grow long and then die off rinse repeat) or built up mulm.  Don't know how this is possible considering the plant mass in the tank and it hasn't happened before  3. A pH crash from built up mulm/leaf litter and a lack of kH.  I admittedly don't understand pH very well but I've always been paranoid that with my low KH water and use of leaf litter, I might experience a pH crash at some point.  I should probably start using crushed coral like Cory is always recommending.  4. Parasites.  I lost 3 male corys 6 months ago.  I'm pretty sure they were outcompeted for food but I always suspected they had parasites. They were very bad at eating compared to the females.  Has anyone had this problem before?  They lived for 5-6 years but were always borderline apathetic towards food.  5.  Disease. I don't know of any disease that would kill so fast and manifest NO cosmetic symptoms whatsoever.

So that's it. I love these fish and it's very discouraging to lose them.  They seemed so healthy.  Any help is appreciated!

Edited March 12, 2023 by NCfish

[Theplatymaster]

@NCfishplease test the water and post parameters.

 

Edited March 12, 2023 by Theplatymaster

[NCfish]

@Theplatymaster I guess I kind of buried the parameters...

On 3/12/2023 at 7:27 AM, NCfish said:

 

Tank parameters: 20 gallon long tank 3 years old with sponge filter.  Lots of water lettuce and water sprite.  1/4 inch pool filter sand and some leaf litter (magnolia and oak) pH 6  GH 3-4 KH 0-1  Ammonia 0  Nitrate ~25 Nitrite 0  Temp 71F

Tap water parameters as of 3/11  pH API says 7.6 or higher.  Co-op test strips read 6.4 so not sure what that's about. GH 3-4 KH 2-3  Chlorine at least 1.5 ppm

 

[Colu]

I think it more then likely caused by chlorine in your water what I would do is holed off on your water change or do smaller less frequent water  if you have high levels of chlorine in your water as long as your not not seeing any ammonia or nitrite and add a add and extra air stone and you can dose up to 5 the amount of prime in an emergency I would do that once then add a double dose daily till water company has finished flushing the pipes and you not seeing any chlorine in your water @NCfish

 

 

Edited March 12, 2023 by Colu

[Mary W.]

Maybe an issue with oxygen level in the tank?  The water lettuce is covering a lot of the surface - 

[nabokovfan87]

On 3/12/2023 at 4:27 AM, NCfish said:

I added an airstone to offgas any residual Chlorine on Thursday night after water change.

Awesome that you added an airstone.

On 3/17/2023 at 9:33 AM, Mary W. said:

Maybe an issue with oxygen level in the tank?  The water lettuce is covering a lot of the surface - 

Floating plants can/will limit gas exchange if covering too much of the water surface.

On 3/12/2023 at 4:27 AM, NCfish said:

I have good video of symptoms but don't know how to upload a video.

upload it to youtube under "unlisted" and then post the link here.

On 3/12/2023 at 4:27 AM, NCfish said:

Tank parameters: 20 gallon long tank 3 years old with sponge filter.  Lots of water lettuce and water sprite.  1/4 inch pool filter sand and some leaf litter (magnolia and oak) pH 6  GH 3-4 KH 0-1  Ammonia 0  Nitrate ~25 Nitrite 0  Temp 71F

Temp seems slightly low, so I would just verify it is indeed 71 (and not lower than 68) due to the indicator being off.  I try to target 72-74, but 71 is absolutely fine.

PH seems pretty low.  I would opt for trying to raise KH, increasing PH, but I don't know full scope of this tank.  For most fish you'd want to keep the PH above 6.5 and for corydoras below 7.5.

Planet Catfish lists 6.0-7.5 which is within range.  I don't know how perfect of a resource that website is, but I think it's the best one available for corydoras.

https://www.planetcatfish.com/common/species.php?species_id=271

On 3/12/2023 at 4:27 AM, NCfish said:

My current theories are;  1. There somehow was residual chlorine in the water even after aerating 3/4 of the water for 24hours and double dosing prime.  A few of the gallons I added during the water change were not aerated overnight but still dosed with Prime.  I've read Corydoras can be especially sensitive to chlorine. 2. A slight ammonia spike from water lettuce root dieoff (which seems to come in cycles. They grow long and then die off rinse repeat) or built up mulm.  Don't know how this is possible considering the plant mass in the tank and it hasn't happened before  3. A pH crash from built up mulm/leaf litter and a lack of kH.  I admittedly don't understand pH very well but I've always been paranoid that with my low KH water and use of leaf litter, I might experience a pH crash at some point.  I should probably start using crushed coral like Cory is always recommending.  4. Parasites.  I lost 3 male corys 6 months ago.  I'm pretty sure they were outcompeted for food but I always suspected they had parasites. They were very bad at eating compared to the females.  Has anyone had this problem before?  They lived for 5-6 years but were always borderline apathetic towards food.  5.  Disease. I don't know of any disease that would kill so fast and manifest NO cosmetic symptoms whatsoever.

1. you can dose prime once every 24 hours if need be.
2. you have floating plants, and a good amount, so I don't think ammonia is the cause here.
3. likely not a good factor here, but might not be the full scope.
4.  what sort of parasites?
5.  would need to see the video to get a feel for what is going on.

Edited March 19, 2023 by nabokovfan87
added link