your refractometer is not good as its meant to work with oil\vinegar\cooking salt rather then sea water (its ment to be calibrated 20c as you can see - Lab environment )
sea water refractometer is meant to be calibrated at 25c (same as our aquariums water temp), so your salinity readings are probably off.
you need to buy a good sea water refractometer such as the one red sea manufactures, then you need to calibrate it to 0ppm with RODI water at room temperature.
i bet your salinity is wrong which causes various parameters to be wrong which leads to chain reaction on few other things add the algae that bothers the zoas , you have a total mess.
the basic is salinity , you go from there and work your way up.
also judging by your lights seems like you have tons of whites - algae loves it and thrives , reduce these whites to contain algae.
Hi slapman83 and thanks for your hints
I use that refractometer for almost 2 years now and as it says it’s a auto temperature adjust I always thought it would be ok to use it here
And that 20c we are in the display, according to the instructions is the desirable ambient temperature of the room when we recalibrate the refractometer
I randomly check salt with a densimeter, also making the temperature conversion and it matches with the refractometer readings
When I make salt water I respect the Red Sea g/l of salt and in the end it matches with the readings
And I do all this for about two years now, and I never had an issue like this one
Regarding lights, I took the photo with an IPhone raising up the whites to try and have a better sight of the purple algae as they get dark on photos if taken with no adjustments
Now what I think it has caused this and that was what went different from this all two years :
I had a busy period that didn’t allow me to make water changes for almost one month
For the first time I could see with my eyes that the water wasn’t cristal. Even the glasses started to get light green/brown with glass algae
And also for the first time I got Nitrates above 20
And it seams I am getting Cyanobacteria for the first time in a tank corner
Photo taken with light adjustment
No light adjustment
Not forgetting that my tank is a 4,5G tank, so any variation can unbalance the system
Now, I made a big water change and cleaned the tank and filter system
And let’s see what will happen