Could somebody help me identify what's going on with some of my corals?

Stelioshah

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Hey guys, another post again but I am really puzzled. Over the last few weeks, a zoa frag a cyphastrea frag and sometimes an acan frag look stressed. Parameters currently are:
Alk: 8.5 dkh (stable for the past month), Ca: 430ppm, Mg: 1350ppm, Nitrate: 10-15ppm, Phosphate: 0.015-0.05ppm, Salinity: 1.026. As you can see in the pictures below, some corals are extended as usual (even my struggling platygyra is finally extending feeders during the night), while others either totally closed up, or stressed with their mouths wide open. The cyphastrea has had its mouths open for at least 1-2 weaks.

Following, I am going to list all of the changes that I have made, along with everything I have noticed changing in the aquarium. Ever since summer started, I started getting way more water evaporation which in essence meant more topping off. That lead me to believe that my RO water was bad. I got a tds meter right away, but it read 0 tds. Despite that, I changed the RO filters, and did a water change. I am also running active carbon in a canister filter 24/7 so I was technically safe in case any chlorine escaped the RO (which would not give a TDS reading). Nothing changed after the water change.

I am going to list changes in the reef scape now. During the past month I have finally started getting some pink coraline growth on the glass and on the rocks. Along with the coraline growth I also noticed that I was getting a lot less brown algae on the glass and some green algae made its appearance mostly on the sand. The lack of brown algae led me to believe that my phosphates were extremely low (tank is 8 months old). Indeed, after dosing some phosphates and bringing them up to about 0.015-0.05ppm, the zoa frag extended within a day (not as much as it used to, but fully open polyps) but the cyphastrea kept staying with its mouths wide open. Today, the zoa frag has closed up again (it has probably been something like 5 days ever since I dosed the phosphates). I am also getting some insane green-hairy algae growth on the sand. I clean the sand algae in the morning and at night the sand is all covered with green algae again. The stress on corals started before I had this much gha on the sand.

The salt I am using is tropic marin classic. My lights are probably not the issue because I have had the corals under the exact same lighting settings for months and the corals were fully extended. Could the algae be outcompeting the corals leading them to stress? I cannot understand why not every coral is affected. Day by day I am getting more and more coraline algae but along with it more and more coral stress. In the past I have noticed that after cleaning the algae and doing a water change that corals would show some relief, this does not seem to work any more. I do not notice any pests on the corals as I dip everything. The zoa frag you see totally closed in the pictures below, used to remain open all night long. What would you do in my place? I really need some help.

 
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Timfish

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Try keeping your phosphate a little higher (FWIW most of the ocean sits around .2 mg/l and upwelling and currents will expose corals to those levels). It sounds like you're getting cyano growing on the sand, I'd siphon it out with the top layer of sand, soak in H2O2 for a bit, rinse and let sit for a day or two before returning to your tank. Research has shown significantly different responses to environmental conditions at not only species but genotype level so seeing different responses between different corals shouldn't be a surprise.
 

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