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Only my Corydoras habrosus keep dying (potential swim bladder problem)


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Hello everyone,

after watching Corys livestreams regularly, and after researching myself online but not finding a solution, I hope someone from here can help me.

I have a 350L (92 gallons) community tank with the following fish:

  • 5 young angelfish
  • 2 honey gourami
  • about 25 rummynose tetra
  • about 30 amano shrimp
  • 12 Otocinclus
  • 9 Panda Corys
  • 3 different kinds of nerite snails
  • 20 (now maybe 12) Corydoras habrosu20210216_204200.jpg.9565d862d113cad11576d7167243aa8c.jpg

You can see photos attached, it is a planted tank that is now running for around 4 month.

Water parameters: ph 7.5 / gH 12-13 / kH 11 / NO2 0, NO3 maybe around 10 mg/l before a water change / temperature 26-27°C (around 79 F)

I do a water change of 30% every 1-2 weeks, I usually follow Corys advice to rather test the water regularly instead of overdoing it with the water changes, I also have a decent amount of plants, I think/hope especially my floating plants help with the NO3.

I have to say that all animals except the Corydoras habrosus seem to do very well: My two larger angelfish already layed eggs once, my Panda corys show breeding behavior, my rummynose tetras are bright red, basically all my female amano shrimp carry eggs, my honey gouramis also build bubble nests, the otos have a nice, round belly.

Only the Corydoras habrosus keep dying. You might think the angelfish attack them because of their size difference, but this is not the case. When feeding pellets etc, the angelfish eat on them side by side with the corydoras and never get aggressive in any way. They also don't care about the amano shrimp, which are relatively large already. I started with 12 habrosus, as they are my favorite kind of Cory. In this phase, maybe 3 or 4 died. I suspected my very aggressive apistogramma agassizi female, so I moved the apistos to another tank. After that, the dying stopped for a few weeks, so I bought new ones to get up to about 20, which was my original goal to have. Now, probably 8 or so have died in the follwing pattern:

On day one, one habrosus swims around the top of the aquarium a lot, and seems to not want to or be able to get to the bottom of the aquarium. On the next day, it is dead. I think I have lost one fish every day for the last few days, today it was 2 at once. Yesterday, I saw one unable to control its body while swimming. I think this speaks to a swim bladder issue. What I don't know is why it is only affecting the habrosus, any why only one or two at a time (all the others seem fine at that point, until one of them starts with the same behavior). They eat normally with the other fish. I have to say that I don't have sand but round gravel in the aqaurium, but I don't see them trying to dig into it to get food. They mostly chill on plant leaves when not eating etc.20210216_092019.jpg.b165732f6e2152219ff47b9f839526d6.jpg

I got all of them from the same source, it assume they might be wild caught so maybe they are use fragile? Another theory I had, I have an O2 diffusor in the output of my filter that adds air bubbles similar to an air stone. I read that maybe these small bubbles destroy the swim bladder of those small fish? Or maybe my tank is too high / the pressure is too high for those small fish? (around 25 inches high). I would really like to save the remaining habrosus, but I also don't want to do large changes in the aquarium that could distrub the other fish that all seem to do fine. I reduced the O2 diffusor to produce less bubbles a few days ago, but since then 3 still died (maybe the damage was already done if that was the reason). Really I would just like to know if anyone has any alternative idea what I could do or how to find out the actual reason for their problems.

Please tell me if I should post any further information.



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What is your ammonia level corydoras can release a toxic secretion when stressed such as netting and transport in bag that is usually fatal was it the new ones that died or one from existing group and how long after you add then did they die

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I just tested Ammonia, is it at 0. I cannot really differentiate between the ones from the first group and the second group, but you might very well be correct that it was only the ones from the second group. A little additional data point: I actually bought a single habrosus after I bought the second group, because it was the only one at the store left, and that one still lives.

I bought the second group around 1 month ago, and I would say maybe two weeks ago they started dying again, up until at least today. I think the timeline was similar for the first group, they didn't die immediately after I got them.

When I bought them the first and the second time, it was at least 12 in one bag, and they were in that bag probably for 45 minutes total. My method for adding fish to an aquarium is from the king of DIY where I first float the bag in the tank for temperature, and then put a net over a bucket, open the bag and dump the fish + water in the net, then adding the fish directly in the tank without the water from the store. If the corydoras released ammonia in the bag, the floating time was probably a bad idea, even though I don't remember them panicking inside the bag.

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I had a similar situation last year when I was running a diy co2 system.  Some fish would swim around seemingly in a frenzied uncontrolled manner.  Often in circles from top to bottom of the aquarium.   The cherry shrimp we’re doing something similar.  But the guppies seemed fine.  
I quickly became suspicious of the co2 system because the needle valve was not staying stable and the flow of co2 was fast then slow and needed hourly manual regulation.  
I disengaged the co2 system completely and put in an air stone on an air pump.   The deaths stopped within 12 hours.  No new cases arised.  Unfortunately all the cherry shrimp were gone at that point.   
I have not since restarted the co2 system.   And though my Amazon swords are not as quick growing as before, I’m no longer loosing fish 🐠 either.  
Thank goodness for root tabs👍

If your fish are making it into the aquarium live and healthy looking I seriously doubt ammonia poisoning is a factor.  But if you are concerned about this you could always insert an air stone into the bag while it floats for temp. I do this for large fish or bags with many fish in them.  

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Do you quarantine them? If so, do you do any treatments for them? As you mentioned, these guys are often wild caught, I just wonder if they are introduced to some new pathogen when they enter your tank and because they are just little guys, they succumb faster than some of the other, bigger fish.

I love habrosus too; I've had them twice. The first time they were absolutely thriving in my setup.  I had to move, so I sold them off, then got a new batch at my new place. Weirdly they just kept disappearing one by one. So... at least know that you're not the only one with this issue. I'm not sure exactly what it is but I think they can be a little sensitive.

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I don't yet have a quarantine setup, I started with the big aquarium and basically got most fish from the same 2 sources in a relatively short period of time, so I wasn't too worried. Now, I wouldn't add any new fish directly into the tank, so I'm currently setting up a small quarantine tank. Therefore I did not medicate them. I would agree with you that there is something bad with the tank in general, if it weren't the case that all the other fish aren't doing just ok but very well. For example, the last time I had Otos, 80% died, as I read is often the case. Now, all 12 are still alive and very active and well fed. The angelfish just now layed eggs again, etc.

Maybe a new possibility: I just found a habrosus stuck in front of the filter intake, which actually already has a "shrimp safety" cover making the holes smaller. Now I'm not sure if it got stuck there, because it is weak and dying, or if it got hurt by the intake and is now weak because of that. It is the first time I've seen this. It was able to get away from it though, I will monitor the little guy but it doesn't look too good.

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