HELP! coral tissue is behaving strange!

[Mooose]

I have have some problem with some of my sps.

Om loosing tissue on random places.

Tank size 800 litres.
I cant se any pests. Have checked.
Flow rate is about 30.000 l/h
Light t5. Bulbs are old. 13 months. But i doubt thats the reason.
Temp 28c
Ca 400
Mg 1380
Kh 7.2
K 380
I is ok
Phosphate 0.11 (a bit high)
Nitrate 1

Here comes some photos.

 

[lilchris_357]

I would say to start looking for pest, and dip a few of the corals that has issues.

 

[Mooose]

I would say to start looking for pest, and dip a few of the corals that has issues.

I have done this, there is no pests to be found, have dipped, looked with magnifier, lights, u name it.
No eggs, no buggs.

Also corals is showing no STN or RTN from base, just on radom places on the branches.

 

[nautical_nathaniel]

+1 on looking for pests, almost looks like some sort of infection too. Iodine dip or something like CoralRX would help with that. I've used ME Coral Wash Off Dip (free samples at MACNA :)) and it worked really well for these types of issues, so there's another option.