Platy dead this morning

[B75]

I came down stairs this morning and was shocked to see one of my platies dead, what I don't understand is that last night I spent about an hour just watching them all swim round and they all seemed fine. I checked my water readings this morning and they are the following

Ammonia 0

Nirtrites 0

Nitrates 25 ( always have been)

Ph 7.5

Temp 77f

The only thing I did was a 20% water change yesterday morning using stress coat before adding the water to the tank. Why did my platy die?

 

[gina d]

just wondering when you bought the platy, is it relatively new to the tank?? due to the fact that the water seems fine i just thought that maybe there was parasites present when you bought the platy from the store ( take note that i am very inexperienced )

 

[B75]

This platy had been in my tank for about 7 weeks. Nothing has been added to the tank since I got the platies.