Problem with my Platies

[afullerton]

This week I lost 3 platies and 2 cory cats. I have been doing weekly water changes, except for last week when it was missed because of work. I have been checking water with API test kit. Ammonia is between 0ppm and .25ppm as best as I can guess by color chart. Nitrites 0ppm and nitrates about 40ppm.
Did about 50% water change today. Added prime and stability. One platy still looks kind of lifeless.
My tank is 60g with 306 Fluval filter and 2 airstones. Stocking level was 15 platies and 4 cories.
Any help or suggestions would be greatly appreciated

 

[Delapool]

Hi, generally to cross fish species (and have rapid deaths), I find that can be from water chemistry or bacterial infection. Water chemistry could be used wrong bucket from shed or water company changed water (just putting them out in case something helps).

 

[aqua_chem]

Can you tell us more about the tank? Particularly the age and if there's been any recent changes to the tank such as new fish?

Also, pictures can sometimes be helpful in cases like this.