One of my platys just died, what do I do

I was looking at my tank this morning and noticed that one of my platys wasn’t moving so fearing the worst I started moving things around it and after a while I established that it had died. I don’t know exactly when it died but I know it must have been some time today. I have only had them for about a week now and just a few days ago they had started eating after refusing to even look at the pellets I was giving them since I got them. I don’t know what could have caused it to die can someone please help me and is there a possibility that the other fish could be affected by whatever caused this fish to die.

*extra notes
the tank is 127 litres with 3 angelfish and 5 platys and I have been testing the water once a week
PH 7.4-7.8(it is very hard to read and was stable at 7.8 for quite a while now until I tested it today)
KH 4
GH 10 (was stable at 9 for a while until I tested it today)
ammonia 0-0.25
nitrite 0
nitrate 10-20

 

Best thing to do is a 50% water change and keep going. Could have been any number of things why it died, happens often in the first couple of weeks coming from the store.

 

Ok, thanks for the reply. One more question, one of my angelfish is always hiding behind a plant at the bottom of the tank and about every 10 minutes he will swim out and then go back, but the other 2 angelfish are completely fine. Also the one that is hiding was hiding behind the filter when I first got them and eventually built of the confidence to swim at the front but about a day or 2 after getting some the new plants he has been hiding behind them all day, is this normal or is it something else.

 

Sounds like possible stress. The others may be picking on him when you aren't watching. How big is the tank?

 

The tank is 127 litres and I have noticed that one of the angels has been chasing another one but as far as I am aware the one hiding has nothing to do with it.

 

I would probably rehome 1 of the angels. They need a lot more space for 3 of them.