SPS doctors, please diagnose

[DBSAM]

I have had this coral for 6 months with decent growth...
I recently changed over to T5 from LEDs, about 6 weeks ago. It was bright green and during the light acclimation time it turned brown. I had an alk spike from 6.6 to 8.0 of 48 hours, but was losing tissue before that...
Alk is now stable at 7, PO4 0.00, nitrate 0...I am using NP biopellets for 12 months.

I cannot find any possible pests.

My first question, is this the result of the alk spike?

 

[Reef Bass]

Alk spikes usually burn tips mostly, so judging from the picture, I'd say that's not alk spike damage. And you say you were losing tissue before then, so again, probably not directly caused by your alk spike.

 

[DBSAM]

low nutrients? pests?

It definitely did not start at the tips.