Ill Platy?

This morning we transferred my two platy's (bought from Pets at home) over to their new, and bigger tank. The new tank is 24L.
 
I added new gravel and new water but I'm using the same filter, which was in my old tank as I've heard this is what contains the bacteria that the fish need..
A couple of days before transferring both fish, one of them decided to start to hide behind my filter - I put it down to them wanting somewhere dark to hide, but I wasn't sure. However, I decided to go ahead with the fish tank upgrade and I put both fish into their new tank this morning. The one that hasn't' been hiding seems fine with it - exploring it, but the fish that was previously hiding has now resorted to laying on the gravel at the bottom of the tank, and not really swimming. The other fish is staying near to the fish that's laying on the gravel. 
Also, the healthier one looks a lot more plump - which I'm pretty sure there meant to be. The ill one looks very thin, almost flat.
I am undecided on what to do. I am concerned that the fish lying on the gravel is ill and is going to spread the disease to the healthy fish. 
What should I do? If you need any information just ask and I'm happy to reply : just want my fish to be okay. 
 
Here is some pictures:
This is the ill fish.
This is the healthy fish.
This is both of them next to each other. 
 
thanks :]

What are you water parameters? (pH, ammonia, nitrite, nitrate) What size tank were they in before? How long? What is your water change schedule like? (How much & how often)
 
Once you've gotten that one feeling better hopefully, I recommend upgrading. They need a 56l minimum.

They were in a 15L before (extremely bad I know.), for 6 months, and I change 50-75% of the water every week to ensure the tank's clean..However I guess the last couple of weeks I've been slacking..so the water quality did get a bit bad..They are in fresh clean water now. 
I will go check the water parameters right now..
 
I regret keeping them in that 15L.

Ammonia: 0
pH: 6.7
KH: 190
GH: 400
Nitrate: 25
nitrite: 0
 
I'm using the King British strip tests. 

It could be anything, but if I were you, I would separate the two fish, whatever it is, you don't want your other fish catching a disease

Okay thanks. Anyone else have any ideas on what could be wrong?

Just went to check on the platy. Poor fish is STILL laying there - it's been around 3 hours now and they haven't moved..is there anything I can do? Would the fish medicine stuff you get in Pets at home be any help? Willing to go get it now..

Fish still laying there, every so often the tail moves as if they're about to swim off, but nope, it's just the tail moving...I feel so bad because they could be suffering.

Any replies? Sorry If i'm spamming or something, I'm just so desperate for some advice right now.
 
I think my lil fish is gonna die soon, just laying there, trying to swim, tail moving but getting no where..help please.

Hi, sorry to hear about your fish what is the temperature of the tank? it does sound and look very ill I don't think there is much you can do at this stage tbh just keep the water clean and warm, perhaps add a bit of aquarium salt and see how he/she goes? could be internal parasites perhaps? 

Thanks for the advice everyone but sadly I was too late. I separated both fishes last night and put the ill one in a different tank, poor fish went straight to the bottom of the tank,lying on the gravel..
Woke up this morning and poor fish has gone to fishy heaven. R.I.P Lilo. X

Sorry for your loss...