Understanding why mushrooms, green star polyp and zoe die but LPS thrive

[PRC]

I am hoping to get a little help.
Lost zoeanthids, mushrooms and green star polyps - were thriving for about a year- zoeanthids grew from 4 to 23 (these cycled like this oftern - they would grow and then die off) and they all then died (melted) within 1 week. Same with green star - just started closing and never opened again.

• Duncan - 5 separate groups - just fragged - HUGE and growing,
• Moon brain (favite) - pretty new - good color, seems to be doing well
• Frog Spawn - pretty new but looks good, bubbles extended
• candy cane coral and Green trumpet - both doing well - almost lost them to a bacterial infection months ago but doing well now
• toadstool mushroom leather - doing well
• green long tip/rose bubble cross anemone - 7 -they keep splitting and growing (had 2 more but traded for coral)

Parameters
125 gal tank with R200 sump (minimal cheato - keeps dying), octopus 160 skimmer, carbon in filter sock, T5 lighting - 12 bulbs 4 months old and 1/2 5 months old
Ca 440 - Red sea & API
Mg 1200 but adding - supplementing - Red Sea
KH 10 dkH - may be 9.5 (add every 2-3 days depending how how much evaporation) -Red Sea & API
Nitrate - stay around 12-15 Red Sea
Phos - pretty sure I have some even though I had GFO in filter (with nitrates at that level I quit testing)
1.024-1.026
temp usually 80-82 (hard time lowering)

I am having issues with algae -green, hair, brown, cyano....pretty much all - I cut back on feedings - 4 frozen cubes (mysis, brine, spirulina - all rinsed and cyclops) that last 11 feedings 9 fish (1 is a tang), 2 shrimp, hermits and variety of snails. Using AlgaeFix (started after the corals died) and it has been 1 month and algae is worse - waterchanges every 2 weeks 10-15 gals

QUESTTION: why would the corals suddenly die? Is it long term higher nitrates? Why do the LPs seem to do well but the soft corals die? It seems like they grow like crazy for a long time and then suddenly die. Open to idea, suggestions, help!
thank you.

[Dkray944]

Cyno can easily kill zoas if untreated. I'm thinking your algea might be competing for food with the softies and cheato . 3 day lights off period works like a charm to get rid of cyno.

[PRC]

How do corals and anemones do with no light for 3 days?

[Dkray944]

I don't have any anemones so I'm not sure about that. In the past when Ive done it my corals were fine.

[quicksilverz]

3 days with out light is basically starving them. I would be very careful about going any more than 3 days. And yes the Cyno is competing for food and starving them.

[kkiel02]

Id test phosphates or have a lfs test for you. I had the same problem as you but it was my magnesium. It was below 1000. I'm still battling hair algae atm.


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[klwheat]

I've done the 3 days no light...only issue I had was that when I kicked them back on at full strength, a couple of my corals (mainly my flower pot) was not happy. Reduced the lighting and ramped it back up slowly...no issues.

[Bill S]

Take a sample of your water and all of your test kits to a LFS, and both of you test your water. Wouldn't be the first time a test kit was off.