Hi guy's

I changed from marine to a planted discus tank 5 months ago.It is a 150 litres and I started with 3
and was advised I could keep 6. I also added some Cory's and cardinal's. I have had no issues at all and the only difference being one growing twice the size of the other 2.
However I have added my next 3 discus from the same supplier a week ago and have lost one of my originals while the other two have seemed to go downhill. They're hiding in the corner at the top of the tank.
Any advice would be appreciated
Thanks