2 healthy fish died overnight, no obvious reason, ideas? Parameters seem okay!

[SakuraSky]

So my tank has struggled with nitrates/phosphates/ algae for years.
I don't have many fish in my Fowlr.
I just lost my Starry blenny and flamehawk last night at the same time. Both healthy and no sighs of illness, a few years old. Other tankmates include a white-tail bristletooth tang & mandarin dragonet(plus 2 urchins and some hermits)
Water test this morning PH 8.2
0 ammonia 0 nitrite, 2ppm nitrate, phos is 1.0
Salinity is .025 temp is 78
I am however am noticing more alage than normal. (including some cyano bacteria) and the thick brown goopy algae has been covering the overflow and I have to brush the slates everyday to keep the water flowing. Brown will be covering the gravel in 1 day as well. But... what would cause 2 healthy fish to drop dead overnight?
I'm possibly thinking low oxygen from maybe the overflow getting clogged literally overnight everyday? That's the only thing I can think of, I've had worse algae!
it just seems so weird for 2 healthy/hardy fish to suddenly drop dead with no spike in anything. I've had nitrates as high as 200ppms and I've worked hard getting them down but have always had high phos.
The tank is 4 Years old
I have slacked on w/c because me and my family have been sick for weeks. So I think it's been 3 weeks since a water change. But that doesn't really strike me as a reason for fish to drop dead.

 

[MnFish1]

1. Sorry about your families illness.
2. It looks like you have not a huge amount of inverts/clean-up crew (obviously hard to see in the pics)
3. Quick deaths imply a disease - have you added anything recently, any symptoms before dying, etc? Did you QT the fish?
4. The overflow should not be the problem unless there was no water being pumped from the sump back into the tank.

 

[Phlynamjax]

any recent additions?

 

[SakuraSky]

1. Sorry about your families illness.
2. It looks like you have not a huge amount of inverts/clean-up crew (obviously hard to see in the pics)
3. Quick deaths imply a disease - have you added anything recently, any symptoms before dying, etc? Did you QT the fish?
4. The overflow should not be the problem unless there was no water being pumped from the sump back into the tank.

I *just* recently (within the last 5 months) made my tank habitable for inverts. I was keeping the salinity at .18 previously with 0 inverts for a while (I also had a predatory fish that died of old age a bird wrasse that was 12 that I inherited with the tank)
So the urchins have done an EXCELLENT job with algae clean up, but I only have about 10 hermits and a few sandsifting snails. Nothing else yet.
the only new fish I added was the mandarin about 4 months ago otherwise others have been there for years. No symptoms before dying. The only fish I did not QT was the mandarin, I bought him from the bag before being released at the lfs. Otherwise I buy all my fish fully QT from Dr. Reef. Mandarin is still skinny despite eating frozen and tons of copepods in the tank. So I hope I'm not kicking myself in the gut for adding him I know they are difficult to QT but I also researched the chance of him carrying something and found it to be low and took the chance. My tang so far is fine. What type of disease would the mandarin *possibly* carry that would effect the fish in the way with 0 symptoms?
Good to know about the overflow though.

 

[SakuraSky]

any recent additions?

Only the mandarin dragonet that was added about 4 months ago, bought in bag at lfs and did research on previous forums on here because I know they are difficult to qt and I took my chances with what it could potentially carry, I read most of the time they are low risk. Otherwise I only buy qt from Dr reef. But he's also still really skinny despite eating frozen and a ton of copepods in tank
Not sure what he could possibly carry that would kill my fish same day with 0 symptoms though but maybe there is something wrong with it.

 

[SakuraSky]

** update**
The mandarin passed last night
I could tell he was going to yesterday and had been skinny since i got him, he was moving weird and just looked unwell.
So now the only fish that's left is my white tail bristletooth tang which *seemingly* appears healthy for now. Now I'm afraid to add any more fish though :/