Help needed: trying to clean and restart an ignored tank

I have this 10 gallon planted tank with many amano and red cherry, two pea puffer and two tetra ( they were once six but they died, I’m trying to relocate them)

Chihiros wrgb2 as a light and Amazonia v2 as a soil. I was using co2 before but now it’s long gone.

Using an external fluval filter for 20g

Basically I had to ignore this tank because personal problems. I still fed all the fishes and topped with RO water .

As you can see most of the plants died or are in bad condition and algae’s has taken over, I’m not sure if that is hair algae’s or Cyanobacteria that you can see taking large part of the tank.

How can I fix this without killing anyone? I’m afraid to destroy the balance that is there.

Ps the shrimp are thriving in this mess lol