Sick Platy

[sunnies]

Hi, I am new to keeping fish.  Our daughter asked to bring home fish from school, so, I've been scrambling to learn as quickly as possible.
 
One of our female platies has been hanging out on the bottom of the tank for a few days.  Sometimes it swims around, sometimes it hangs out on top of the heater.  When it swims, it seems like it's trying very hard to keep from sinking to the bottom - its fins "flap" very quickly.  Often it seems like it's "tail heavy."  It's much smaller than the other platies - I think it's always been the smallest, but I don't think the size difference used to be this great.  It's shorter in length and height and thinner.  I've tried feeding it peas (slightly cooked, shells removed, smashed), but it's never shown interest in them.  It does eat the flake food usually.  The other fish do not harass this fish as far as I can see.  And I can't see any other issues with the fish.
 
We have a 10 gal tank with 5 platies (4 female, 1 male), 2 baby guppies, lots of pond snails (I think), and an assassin snail.  It has some plants, not sure what they are.  The snails have destroyed most of them, but at least one is doing really well.  I don't think the platies are full-grown.  They're maybe 1" - 1.5" long?  The tank has gravel on the bottom, a heater, a power filter, and a light.
 
We've had the tank set up with all the fish for about 2 1/2 weeks.  The platies came from school, the assassin snail from a pet store, and everything else came with the tank.  I bought the tank from someone who had kept fish in it for a while.  (For transport, he removed the water, bagged the plants, and put the baby guppies in a container.)   I didn't have any bacteria starter stuff, but I hoped the residual water in the gravel and filter would still have the necessary bacteria.  I did use a dechlorinator solution on the water though.  The assassin snail was added about a week ago, maybe a few days before the one fish started acting oddly.  At that time, the pet store tested our water and said everything looked good.  I tested the water yesterday after receiving our testing kit, and got:
 
pH:   7.6
NH3:   0
NO2:   0
NO3:  10
GH:  5 deg
KH:  3 deg
 
Temp is 76 - 79.  (It's not steady since at the moment, I can only plug in the heater or the light.  Filter is always plugged in.)
 
The other fish seem to be thriving - energetic and growing.  Any help with our sick fish is greatly appreciated!  Thank you!
 
 

 

[the_lock_man]

I've moved this thread to emergencies, as this is a more appropriate place for it.
 
I would suspect from what you've said that it's stress. A 10gal tank is a bit too small for that number of platies, long-term, let alone with guppies as well. It's a bit like trying to live on the NY Subway at rush-hour - you can cope with it for a while, but to do so 24/7 is overwhelming.

 

[BerryAttack]

stress, or could be the the spin was injured, are there any swelling and also looking from the top of the tank is the spin curved and also from the side. if there is even a slight curve in any way there is no way to fix it.. so you can wait to see if it gets better, but if it continues to get skinnier then the fish isn't going to make it.
 
look at the food, white and stingy or clear.. could be internal. 
the scales poking out?

 

[sunnies]

Sadly, it was indeed an emergency, though perhaps already too late.  This morning we found the fish dead and covered in snails.  (I didn't realize snails would eat a dead fish.)  
 
It did not have any pineconing.  I don't know if the spine was curved though.  If it did, it was very slight.
 
It could have been stress, and perhaps it was not a robust fish to begin with.  Is it possible that the fish had a contagious disease?  Do I need to worry about the other fish getting sick?  I did remove the dead fish, all the snails that were eating it, most of the other snails too, and did a partial water change.
 
I have been planning on reducing the number of fish, as I knew ultimately, the tank was too small for 7 full-grown platies/guppies.  But since the platies still seemed small and the guppies were teeny, I figured I had a little time to decide how I wanted to do that.  I'm thinking of giving the male platy away and any surviving platy fry.  (All the remaining females are pregnant, I think.)  Is 10 gal enough for 3 platies and 2 guppies?  And will the platies and guppies each have enough company?  I know they are social fish.  I wasn't planning on having guppies with the platies, but they came with the tank.  I also figured there was a good chance they'd get eaten, but perhaps they didn't since the platies were smallish still.  But they survived, and after looking up guppies, I thought it'd be nice to be able to keep them since they're so pretty.
 
Thanks for the replies.