Corydora suddenly died?

[Sonny278]

Hi all,
I had 3 peppered Corys in my tank and decided to buy two more earlier, both slightly smaller than the original three.
All was fine, they were acclimated before being released into tank and then couple hours later were fed and all ate normally. They were all swimming normally, didn't notice anything unusual but then this evening have found one of the new ones just floating upside down at the top of the tank? Would anybody have an idea as to what could've happened? As I say, he seemed perfectly fine earlier when swimming with the others so am a bit confused!
There are two female Betta's and a rainbow shark also in the tank however they have never bothered the Corys

 

[JuiceBox52]

Both bettas and the rainbow shark pose a huge threat to corydoras. The latter especially

 

[Sonny278]

Thanks for your response. I have never had a problem with the Betta's at all, they hardly go near the Corys unless it's feeding time and they're getting food that's sunk to the bottom, but even then no issue with them.
The rainbow shark has only ever gotten aggressive with the Betta's, I've not seen him chase or bully the Corys at all - with current situation I'm working from home so have the pleasure of sitting by my tank all day and can watch them. I guess it's possible he could've had a moment and attacked?

 

[Byron]

You may or may not see aggression, but it can still be present. Fish release pheromones and allomones which are like chemical signals that other fish read, and an aggressive fish (all male Bettas are "aggressive", and the Rainbow Shark is an aggressive species) can still stress out other fish, which weakens them, leading to other problems that otherwise could be prevented. A new fish in any tank will be seen as an intruder, and depending upon the species may or may not develop into trouble.

Male bettas really should be on their own, they are not community fish. Individual fish can have somewhat different temperaments but the species as a whole is still aggressive however this may play out.

Rainbow Sharks should never be housed with cories, the latter will be the losers in several ways.

 

[JuiceBox52]

You may or may not see aggression, but it can still be present. Fish release pheromones and allomones which are like chemical signals that other fish read, and an aggressive fish (all male Bettas are "aggressive", and the Rainbow Shark is an aggressive species) can still stress out other fish, which weakens them, leading to other problems that otherwise could be prevented. A new fish in any tank will be seen as an intruder, and depending upon the species may or may not develop into trouble.

Male bettas really should be on their own, they are not community fish. Individual fish can have somewhat different temperaments but the species as a whole is still aggressive however this may play out.

Rainbow Sharks should never be housed with cories, the latter will be the losers in several ways.

They are female bettas but still are generally semi aggressive

 

[Byron]

They are female bettas but still are generally semi aggressive

I missed that, thanks for correcting. It isn't quite as bad then, but the shark is still a problem.

 

[Retired Viking]

I agree with @Byron your shark is still a problem and I would not trust the female betta either. None of them are really community fish. I would seperate them as soon as possible. Return to the LFS or rehome them to another tank.

 

[Sonny278]

Thanks all for your replies. I definitely don’t want to lose more Cories so I will think about rehoming the shark.
@Byron thanks for the info about fish releasing pheromones etc, I wasn’t aware of this and it‘s really interesting to find out, will definitely keep this in mind with buying new fish