Hammer tissue receding, dark tentacles at night

[itgoeson]

I’m new to reefing and set up my first saltwater tank in February. I’ve had this bicolor hammer since April 4. It’s been doing well for the most part - gets puffy during the day, eats small bits of Rod’s that come its way, new tentacles are growing. However I’ve noticed two potential issues recently:

1. The tissue on the skeleton is slowly receding.

2. At night, it shrinks up a lot and the tentacles get really dark, almost like there is black stuff inside them.

Parameters:
Salinity 1.023 (refractometer)
pH 8.2 (Salifert)
Nitrates 7-8ish (Salifert)
PO4 0.01 (Hanna)
Calcium 400 (Red Sea)
Alk 9.1 (Red Sea)
Mag 1300 (Red Sea)
Temp 77

I have a wall hammer I added more recently that’s doing well so far and some softies and LPS mostly doing well (blasto, trumpet, ricordea, toadstool, red mushrooms, RFAs doing well; acan was doing well but has shriveled up the past few days after eating a lot, hoping that’s a digestion thing and they will puff back up).

Is this normal or should I be trying to move it around etc?

Day:

Night:

 

[Arthacker87]

Salinity seems low to me. I'd say 1.025 is recommended. A swing of .01 is fine. Bmmaybe I'm wrong though. But the top pics look fine really. At least on here they do

 

[itgoeson]

Salinity seems low to me. I'd say 1.025 is recommended. A swing of .01 is fine. Bmmaybe I'm wrong though. But the top pics look fine really. At least on here they do

Thanks! I have the salinity there because I’ve been told it’s better for fish because there is more dissolved oxygen the lower the salinity is. I was hoping I could get away with 1.023 for LPS as a compromise between fish and coral health. There are obviously loads of SPS systems at 1.025-1.026 with healthy fish though. Not always sure what to do with conflicting advice.

 

[Arthacker87]

Haha yea I dunno. My yank always runs at 1.025 and I haven't lost a fish since I set up my 65 (except a firefish that jumped because another firefish dared it to lol) my fish seem healthy and happy. I think fish are way more tolerable to salinity than coral.