tank die off help

Hi Everyone-

First off I am very new and have much to learn. I am seeking advice. Any and all help is appreciated. Please be nice, I'm no expert.

Here's the specs



20 gallon tank been up since October

Water parameters are as follows. I've tested myself but also had lfs check as I wanted another opinion

Sg: 1.025 with refractometer

Nitrites: 0

Nitrates: 0

Ammonia :0

Phosphates: 0

Alkalinity: 7.5 was low when tested now 8.3- this was only thing that was off

magnesium and calcium both tested fine

have 165 w aquamana led light. Its only at about 30 percent as its very strong.

hang on the back aqua clear filter

2 hydor 600 power heads.

temp varies between 78-79 but steady doesn't drop or rise from that.



Tank was fully cycled. I have 3 nassarius snails, 2 turbos and an astrea, and one sand sifting star fish- all appear healthy



2 percula clowns, yellow clown goby.



I had just added a acopora coral and within a day the tissue peeled right off..



My problem other than listed above. I have several soft corals, hairy mushrooms, pink zoanthid, gonipora, mushroom leather that have all been in the tank going on a month or more without any issues. Up until last week all were happy, fully open with no issues.

I had an outbreak of cyano bacteria. Have since gotten rid of it but have a bad case of brown detritus algae, which continues.



As of last week the corals weren't opening. The only ones that appeared happy were the leather. Everyone else is closed. The hairy mushroom who has always been big and happy is now small, every night he exposes this stringy spiderweb like stuff all over itself (some research I've done suggests this is its guts?).It seems to go away and then return. It stays contorted or closes up but never seems to fully open. It is not placed too close to any other corals so don't think its warfare.



2 days ago my larger clownfish died. followed by the second one the following day. The yellow goby appeared fine. Tonight has disappeared so I am going to assume at this point he died :-(

Again checked for ammonia it is fine. Did 5 gallon water change after finding the dead fish and also do about once weekly. Checking levels each time.



My question is what's going on? What am I doing wrong? The red green blasto now appears to have small white stuff on it as well so I fear it is dying and the hairy mushroom also.

My first thought would be something is up with the water, but again the tests are good and like I said I got it checked again via the store to make sure my test kits weren't just bad.



Thanks in advance I'm truly disheartened.

Stef