Help Me Figure Out What's Wrong

[minirhyder]

I had two adult black tail platies in my tank, along with a red tail shark. They reproduced and I had a good number of fry. I didn't do much to help their survival, so I ended up with some 12 or so fry.

 
Fast forward a few months, most of them have slowly died off. The two adults also died, both in one night. The ones that are left (four of them) seem to be stunted, or at least growing very slowly, and as time goes on, their spines are becoming curved upwards around their bellies, which look a little enlarged. When seen from above, their spines also seem to be curving sideways.
 
The shark seems to be doing fine.
There's no other signs of disease.
Edit: forgot to mention, I've got quite a few snails in my tank. They were stragglers on some plants I bought early on, but I've kept them around to eat excess algae.
 
My water parameters:
 
pH: 7.5
Ammonia: 0
Nitrites: 0
Nitrates: 10-30 ppm (depending on day of the week).
 
I do ~15% water changes twice a week (I used to do once a week, but the nitrate levels were getting close to 40 ppm).
I use API's master testing kit, which I bought very recently.
 
Here's the curvature that I'm talking about.
 

 

[Tongue_Flicker]

Looks like some fin/bone malformation/disorder. I used to keep an albino plecostomus with the same symptoms. It was like that already when i got it but i only noticed til it grew big enough to easily observed. They usually live normally like most fish but they don't live as long as they should be living. Some say it's congenital but don't know what causes it as well