Hasbrosus Corydoras - Being murdered? Disease? Something else?

[Kim Nancarrow]

Hello!

I got 12 hasbrosus corydoras and a male and female apistogramma on 02/05/2022 - they went into my 10 gallon quarantine tank to start. Everything was fine until 02/14/2022 - when I lost my first cory. At that time, I started a maracyn treatment just in case. I lost a cory a day from 02/14 to 02/17. Then, everything was fine for a few days (I thought the treatment was working), but then I lost another cory on 02/21. On 02/23 I decided to chance it and move my apistogrammas to the 75 gallon even though quarantine wasn't up. I thought maybe the apistos were doing something - though I never personally saw aggression. I didn't lose a cory in the small tank after that (02/23 to 03/05). 

On 03/05, I moved the corys to the 75 gallon (with the apistos) - as I put a betta in the 10 gallon. There are also 12 cherry barbs, 19 neon tetras and 3 green dragon plecos in the 75 - so, not overstocked. It is fairly heavily planted with lots of hiding spots. I thought that the corys would have enough places to hide and the tank was large enough to allow some separation. 

Today, 03/09, I had another dead cory. There are only 5 left in the 75 gallon. What is going on? Are my suspicions that the apistos are murdering them correct? Should I move the remaining 5 to the 10 gallon with the dumbo betta? 

Water parameters are as follows (they generally stayed about the same with nitrite elevation during the maracyn treatment that I combated with water changes): 

10 gallon - Nitrate 50, Nitrite 0, Hardness 300+, KH 120, pH 7.6, Chlorine 0, Ammonia 0, temp 77.2 F

75 gallon - Nitrate 35, Nitrite 0, Hardness 300+, KH 120, pH 7.6, Chlorine 0, Ammonia 0, temp 76.3 F

There are no visible bite marks, missing fins or otherwise on the little cory bodies. No sunken or distended bellies. Please help me! 

[jwcarlson]

I'd personally move them back to the ten gallon as the "problem" seems to follow the apistos.  I've never had apistos so can't comment on if they can be murderers or not.

[Colu]

It's possible their an underlying disease or its the apistogramma killing them have you noticed any flashing white stringy white poop rapid breathing as most habrosus Cory's are wild caught it could be a parastic infection that why your gradually losing a Cory every so often

[Kim Nancarrow]

Hi Colu - thank you for your response. I am new to corys, so I am not sure about the rapid breathing - I have not noticed any stringy poop though. I lost another 2 last night. I decided to move the remaining 3 back to the small tank with the betta. At this point, I am not sure I can save them no matter what I do. 😢

[Kim Nancarrow]

JW - I took your advice and moved them back to the small tank. I did lose another one before that. 2 left - not the group #s they like, but maybe I will be able to save them. 

[Colu]

What I would do is treat with paracleanse and maracyn together to cover all bases just in case it parasitic or bacterial infection  @Kim Nancarrow