My platy not eating

Hi,
I bought 10 platy fishes 4days back and 5 fishes were dead in the last 3days. Remaining fishes also not eating anything & sometimes all stay at the bottom and sometimes all stay at the top.
I have 5 gold fishes also in the same tank. Currently, 5 gold fishes & 5 platy fishes are there.

Now, my concern is platy fishes are not eating & i am worrying that remaining fish also die. Please help me with this.

Did you cycle the tank? How big is the tank? Platies should not be kept with goldfish due to different water temperature preferences. If you have 5 fancy goldfish you need atleast a 100 gallon tank and excellent filtration. If you are keeping single tail goldfish you need atleast a 200 gallon tank for that many.
And if you can what is nitrate nitrite and ammonia levels

pictures of sick fish?

what is the ammonia, nitrite, nitrate and pH of the tank water?

-------------
Do a 75% water change and gravel clean the substrate every day for a week or until we work out what is wrong.
Make sure any new water is free of chlorine/ chloramine before it's added to the tank.

Add 2 heaped tablespoons of rock salt, sea salt, or swimming pool sat for every 20 litres (5 gallons). Keep the salt in there for 2 weeks.
When you do the daily water changes, add salt to the new water before adding it to the tank.

Thanks for the reply.
I have attached some of the pics of platy & gold fish and the tank as well. I don't know the size of the tank and i didn't check any water levels. I am new to this field.

The orange livebearers look like swordtails, not platies. However, that is irrelevant because they all get treated the same way.

The fish have clamped fins and excess mucous. This can be caused by poor water quality or an external protozoan infection (Costia, Chilodonella, Trichodina).

The best treatment is what I mentioned in my earlier post, big daily water changes and salt. Do the daily water changes for a week and keep salt in the tank for 2 weeks. See how the fish look after that.

Try to get the water tested for ammonia, nitrite, nitrate and pH.
Most pet shops will test water for customers. Take a glass full of tank water to the pet shop and get them to test it for you. Write the results down in numbers when they do the tests. If the shop say the water is fine, ask them what the results are in numbers.

What are the dimensions of the tank? If you can measure it you can calculate the volume of the tank

Goldfish should not be kept with swordtail as they produce lots of wastes

What are the size of the tank?

My tank is 15 gallon