Dying Corals but no phosphate/nitrates

[Zeleriand]

About a month ago I added 3 corals to my tank: sunflower, hammer coral and elegance. They were doing fine but a week ago the corals started to receed. The elegance is almost dead by now and hammer is looking really bad.
Nothing seems to be wrong witht he water, and I had it checked for a 2nd opinion:

Nitrite: 0
Nitrate: 0
Calcium: 410
Alkalinity Kh: 8
Salinity: 1026
Ammoniak: 0,15
Phosphat: .03

Brown algae is growing everywhere, and there is no way to get rid of it. (also some kind of algae is growing on the zoa)

Its a 90 gallon tank, there is a protein skimmer. 2-3 days ago I added caulerpa as I thought it may be phosphate, however the algae are still there and I cant see phosphates on water measurements. Also I had change 10% of the water 2-3 times in the last weeks.
The tank gets light from 10 am to 10 pm, gradually intensifying and then changing into blue at the end of the day.

Any ideas what could be wrong with the tank? I really would like to save those corals.

 

[bang guy]

Can you be specific on the lighting and amount of waterflow?

You listed Ammonia as 0,15 - I'm not sure what that means.

Any other livestock in the tank?

What were you feeding the Tubastrea and how often?

What are you feeding the other corals?

 

[Zeleriand]

About lightning:
The lamp is a Maxspect Razor 130w, 15000 K with 120 lenses. Auto programmed to 12 hours of light to 80% of its max.
The returnpump is a Tunze TUC-1073.040 running 2.500 l/h

The Salifert test set of ammonia says <0.15 ppm

The sun coral wa seed with frozen shrimps (artemia). We started feeding it everyday, as soon as the tank started to get bad, we stopped and only fed it 2 times a week. Thats when we bought the algae for the refugium to lower the phosphate.

For livestock there is:
1 Valentinni sharpnose puffer
1 Hugo (salarias fascistus)
1 halichoeres chrysus
1 foxface (although I haven't seen it for 4 days)
1 bullet goby
1 banded urchin (Echinothrix calamaris)

For corals: some zoas, sun coral, the dying hammer and yellow tip torch and a xenia (picture attached). It seems only the hammer and tip torch who are suffering.

 

[Zeleriand]

Ohh and some turbo snails and hermit crabs

 

[bang guy]

I think there's a couple things going on. From what I can tell you do not have enough waterflow for those corals. I also believe you have excess phosphate but most is being consumed by either Cyanobacter or dinoflagellates. I can't tell which from the pictures. If the red/brown algae you see on the sand and on some of the rocks decreases at night and grows during the day then it's probably Cyanobacter.

The Sun Coral would benefit from some meatier food but at the same time that would make your water quality worse. Not sure how to solve that dilemma.

You may be overfeeding your fish.

I think the first thing to fix is your waterflow. That in itself could solve all the problems you are experiencing.

How long has the system been set up?

Is all the rock live rock or was it originally dry rock?

Did Ammonia get really high during the cycle (only answer if you had some live rock)?

 

[Zeleriand]

Thanks for all the info.

There is also in the tank a maxspect gyre 130xf that produces a max of 2300 gallons per hour.
That works along with the 660 gph from the return pump.

The tank is from May so only 3 months old. The biggest chuck was dead rock but we added 3 kilo of alive rock to kick start the process.

We haven't noticed a spike of amonia but we only started measuring it in the last month.
The algae seem to come and go. It's very dark brown and extends quite long. I'll post a picture of it as soon as I get home.

Thanks a lot