3 suddenly sick Platies?

Hey guys so I suddenly have 3 sick Platys, 1 died who strangely enough was originally doing better then the others I'm gutted because he was one of my Favourites as he looked like a cow his name was Moomoo Apple was the 1st to become very lethargic then suddenly Aqua and Moomoo were too the only thing up is my Ammonia at 0.50ppm which I'm unsure as to why it has suddenly risen. I've had Apple for 1 week today her and Aqua are both very very clamped and their tails are narrow. I added some salt to the tank and have done 2 very big water changes they are in a 90L tank wirh 8 Blue Neon Tetras, 8 Black Neon Tetras, I had 7 Platies (now 6), 3 Mollies, 2 Guppies and 2 Bristlenose which I have had all together for ages. are my other fish going to get sick? they struggle to swim but manage to and are eating. Moomoo tho he was hiding behind a log then when I checked on him again part of his head was under the log I thought he was dead moved it to get him out and he was only just alive and his head and top fin looked like he had internal bleeding so I wonder if he tried to get up and got stuck sadly ): I put him and the other 2 in quarantine where he shortly after died. the photo I have attached was Apple and how she started (I'm starting to think she might be a he as she's very young and has changed a bit since I got her last Saturday) very Lethargic this started Friday and Moomoo died last night it's all happened so fast. anybody know? they also have to put a lot more effort in when swimming I noticed especially Apple.

I forgot to add! when i got Moomoo out I noticed he had almost like a scab on the side of his head. this is Moomoo and Aqua together when I 1st got them both about 4 months ago.

another update this is Apple now she can't swim at all and Aqua i let out as he seemed to be doing ok but can't swim for long before he's on the ground again

I added some salt to the tank and have done 2 very big water changes they are in a 90L tank wirh 8 Blue Neon Tetras, 8 Black Neon Tetras, I had 7 Platies (now 6), 3 Mollies, 2 Guppies and 2 Bristlenose

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IMO your tank is overstocked and you have soft water fish and hard water fish in the tank.

Is your water soft or hard, Mollies do not do well in soft water.

My mollies are fine it's my Platies that are sick all 3 sick ones died as of thismorning. but all the other fish are fine and healthy, I know my tank is over stocked as I had my Mollies and Tetras in another tank but the filter broke and so I added them into the 90L and they seemed to do better in there so I left them and just made sure to keep extra care of the water. I am trying to sell my Tetras though so it's not over stocked. I'm honestly though unsure if my water is hard.