Brown blotches

[Heneee11]

Having issues with some brown blotches on several acropora. Several frags and some colonies seem to develop theses brown patches, anybody any ideas?

Tank set up around 3 years.

maintained using calcium reactor

alk - 7.5.
calcium - 420
mag -1350
potassium - 400

nitrate 5 - 10
po4 - 0.1

salinity 1.026
temp 26c

lighting- ATI hybrid powermodule, tubes up to date. 4 orphek or3 bars

 

[vetteguy53081]

Alk and calcium a little low
Looks like diatoms. Blow gently with a turkey baster and it should clear away