Anybody here with a success keeping an anemone in a low-nutrients reef tank?

I had a healthy anemone for more than 2 years now, it was expanding so much
it occupied 2/3 of hight of my tank. It quickly become a centerpiece in the tank.

I had constant problems with hair algae and red/brown cyanobacteria due
to the overcrowding the tank with fish and recently I started to fight with
this problem with a frequent large water changes to bring the nutrients down...
One time I used very small amount (under dose) of phosban to bring the
phospates down a little...

In the meantime my actinic lamp broke, did not have time to fix it or buy
new one so the tank was under 1/2 light intensity compared to normal...
Also, the only light source was 10000K white tube, no actinic blue.

Other than that, I lost pink sea cucumber (filter feeder one) and I did not
noticed this, so unintentionally I let it decompose between the rocks :-(

Unfortunatelly a lot of changes happened to my tank in the last couple
of months and my tank lost its ballance. Now I have no red cyanobacteria
but the tank is overwhelmed with brown single-cell algae growing everywhere.
Checked the algae under the microsocope and to my surprise the cells are
round and motile. Do not have better microscope to allow identification.

My invertebrates are sick. Pompom Xenia is almost gone. Reduced to white
dots on the rock - almost nothing left. Green and brown button polyps
are shrunk and do not look healthy. The biggest issue I see with the anemone.
My bubble tip, which was the center piece and my pride for so long is
now reduced to about 1/5 of its fully-pumped size and does not accept food.
Clownfish are still hugging it, nurture it, bring food to its tentacles
but the anemone is passive. It does go throught the cycles of being larger
and smaller - it seems to open its oral opening during feeding, but its
tentales do not fire and the food does not stick to its body...

I am worried I could lose it and I am not sure how to help.

I am scared of toxins after the sea cucumber decomposing (I read they
can wipe out the tank when they die...) so I continue doing water changes
but this moves my tank way more out of ballance and the brown motile algae
seems not disouraged. I detect zero nitrates now, little phosphates.
The salnity and calcium is normal (34ppt, 400mg/l).

I am affraid I have conflicting goals to achive now: I want to dilute
toxins from decomposing cucumber, but I do not want to dilute nutrients
for the soft corals. Am I correct I am causing more harm to the anemone
diluting nitrates and phosphates or they do just fine in low-nutrients
environment?

Any help or ideas would be appreciated.

Thank you.