One Platy Dead, Another Dying

[daizeUK]

Previously healthy female platy found dead in a 45 degree angle (head at the surface and tail tilted downwards).  She was born in my tank April last year so she'd be about 16 months old.
 
Her twin sister is currently looking very weak, also floating at surface or hiding in plants with that same 45 tilted up angle.  She looks like she will die soon too.
 
All other adult and juvenile platies in the same tank look healthy and active.
 
I returned from holiday on Saturday to find my oldest fish dead and half-eaten.  She had been showing signs of old age for some time so I wasn't surprised to find she had passed away, I just assumed it was old age.  She must have been dead in the tank for several days.  The tank had a 50% water change on Sunday.
 
Water tested and ammonia and nitrite undetectable.
 
Any ideas?

 

[techen]

Sounds like that old dead fish caused a large ammonia spike when rotting away which killed two weaker fish off and then your filter quickly cycled through what was left of the ammonia hence no trace now.

 

[daizeUK]

I was concerned that the rotting body may have polluted the water.  The thing is all the other fish looked perfectly healthy when we got back, no gasping at the surface or signs of distress, so why would two of them suddenly die at the same time a few days later?  And why the odd 45 degree angle?
 
These definitely weren't weak fish, these were two big beautiful healthy home-bred fish.  Their mother is comparatively much weaker, she's still in the tank and looks fine.  Also some 1.5 cm juveniles that I would expect to be much more sensitive to ammonia are also fine.
 
I think the sister has passed away now too.

 

[techen]

The strange angles is just trapped air pockets inside the fish, Makes them float at strange and odd angles.
 
As for the rest of the fish not dying I would say every fish is different, No two fish have the same immune system and being home-bred they will be weaker than most massed produce fish, Why? Simply because in your tanks they've never had to deal with ammonia or nitrites where in stores and being mass bred there's always ammonia or nitrite or any nasty things floating about they have to deal with and they adjust and deal with it.
 
I think you just got unlucky that both of them couldn't handle the slight increase in ammonia 

 

[daizeUK]

Maybe - it's sad but I hope you're right, my first thought was I was facing a sudden disease outbreak such as columnaris.  Thanks for replying.
 
The sister is still barely alive but looking very feeble.  I found her upside down at the bottom of the tank then she swam around a bit and came to rest in some plants again.

 

[Mamashack]

Sorry for your loss, daize, it's hard losing fish especially when you've bred them in your own tanks.
Hope it is the ammonia spike from the decaying fish whilst you were away and nothing more sinister.

 

[daizeUK]

Thanks Mama.  They were the first babies to be conceived with both parents in my tank so they were a bit special, I had hoped to enjoy them for a good while longer.
 
The weak fish has disappeared so I'll probably have to hunt for her tomorrow morning.  Not looking forward to that, it's a total jungle!