Dying Molly Fish!! URGENT!!

[imamesshelpme]

I have three molly fish, two female, one male. I think it's a 17L tank, amethyst gravel, one plant that looks similar to a fern, and a fake log. Water parameters are always fine in my tank, and I currently have no way of checking them.
One of the females is sick. She was 100% fine yesterday, now she's laying at the bottom of the tank, struggling to even hold herself up. When she does move, she just flops. I thought it was swim bladder issues, but she just tried to move frantically, and did about five barrel rolls, which is what scared me.
I did a 25% water change, added a little AQ salt and held her gently and fed her some pea.
At the beginning of writing this, she was alive and fighting, she has now sadly passed in front of me. What could have caused such a sudden death? What can I do to make sure the other two don't suffer the same fate? It might be worth noting that I have recently had two other sudden deaths of mollies in the tank.
Any advice will be much appreciated.

 

[nirbhao]

The best thing you can do right now is test your water. The most accurate, cost effective way to test are liquid like the API master test kit.

Fish deaths can cause an ammonia spike, which can cause more deaths, which can cause more ammonia, etc.

Do another water change. Use a water conditioner like Seachem Prime.

Sorry for your loss. Let's get things healthy again!