Brain losing color + brown spot

[DexterOne]

I've had this brain coral in my tank for 2.5 years and in the past 2 weeks, it started to lose it's colors. it was a vibrant red and green before and now half of its flesh is turning white. There is also some tiny brown spots on it (and noticed some on my chalice also...). He still inflates and open is mouths and tentacles but it's really fading out. I never moved it and besides adding 1 or 2 SPS frags lately, nothing as changed in my tank. I recently changed my Giesmann T5s (around 6 months old) for ATI T5s hoping that that would be part of the problem (but getting tired of my old bulbss ). The tank is 3 years old and is controlled by a controller (RKL)

Parameters :

Salinity 1.025
Temp 78.5
TDS meter on distilled water : 0
Amonia, nirates, nitrite.... don't have a clue (no test kits at that moment)
Calcium : around 400ppm
Mg : ?

I do a 25% water change every week or 2 weeks with the same brand of salt (reef Crystals).

Anyone has an idea on the cause of that color change?

 

[GPB]

You need to make sure your Alk and Cal parameters are in the right range and stay that way. Your tank looks nice. Good coralline growth. I doubt NO3 and PO4 are your problems. I keep my trachy under an overhang and out of direct flow. Seems to help.

 

[DexterOne]

Thx for the compliment on the tank!

I will get a new test kit for Alk and see how it turns out. What is kind of weird is that I have had the same maintenance schedule for the past 6 months and nothing as really changed in my tank.

I would have tried to move it but the tank is getting a little crowded

Anyone has a different hypothesis on what happened?

Thanks.