Euphyllia dying, please help!

[Copernicon]

I've had my Euphyllia for 3 years now and 3 weeks ago suddenly the heads started dying one by one (no brown jelly visible, looks more like white slime). After a couple of days of heads consecutively dying I removed the entire colony, dipped it in AF protect dip and removed the dying (dead) heads before fragging it in two and putting it in 2 different spots. Did a 50% waterchange and changed out the carbon. No problems ever since, until today the same started happening again and I'm afraid it's gonna happen again. I'm at my wits end, please help! (attached 3 pictures before the intervention 3 weeks ago and one picture today)

Parameters:
Ammonia: 0
Nitrite: 0
Nitrate: 5
Phosphate: 0.1
dKH: 9
Calcium: 430
Magnesium: 1350
Salinity: 1.025
Temp: 25.5

 

[vetteguy53081]

It’s stress likely from either high phosphate or light and flow
Moderate light and water flow and assure light not suddenly bright
Also check salinity