Need help with green water algae

[supertech56]

I'm pulling my hair out trying to solve a green water algae issue that's been effecting my tank for about a month and a half, now. I'm almost ready to give up on the tank. It has been set up for about 3 months now. It's a 20 gallon, Fluval 107 canister filter, Twinstar SM600 III which is now currently on 5 hours a day (reduced from 7), 30PPM of CO2 dosed an hour before lights on to half an hour before lights off. A few weeks ago I did a 3 day blackout on the tank, with 50% water changes on the first and third day, the GWA came back a day after I turned the lights and CO2 back on. I was on a standard estimated index fertilizer dosing routine, which I'm currently completely avoiding until I can get this figured out.

Tank is currently inhabited by 4 otocinclus catfish, 10 neon tetras, 40-50 red cherry shrimp, and 2 amano shrimp - currently seeing a lot of RCS die off because of all the water changes I'm doing.

It's well cycled, ammonia and nitrites stay 0 all the time, nitrates barely register above 0ppm on the API test kit, phosphates stay 0 all the time. Water out of the tap is extremely soft where I am, 7.2ph, 2kh, 2gh, ammonia and phosphates out of the tap are no more than 1ppm.

What's really odd is that all of my heavy water column feeding stem plants have been displaying symptoms of phosphate deficiency (slow new growth, darkened older leaves, weak stems, spot algae growth). If there's not enough nutrients to support plant growth, surely there wouldn't be enough for algae to grow?

Really struggling to figure out what to do next. I want to avoid going the UV sterilizer route, and I don't really have room for one anyways. I'm planning on doing another 3 day blackout, and reducing feedings to once a day, but I know this is just going to cause more stress to the inhabitants.
I can take some pictures and post the month's worth of diligent water parameter measurements I've organized if it will help.

 

[Azedenkae]

A number of things.

Algae post-death releases nutrients into the water that is broken down and can be re-used by new algae. 50% water change removes half of this, but still plenty left for algae to bounce back super duper fast right when they have a chance.

Nitrate is low probably because it is constantly consumed (by algae).

Algae grows far more rapidly than plants (generally), so chances are they'll soak up all the nutrients before it can be available for plants, thus the plants then die off.

The solutions you did are often thrown around, but are very temporary (and detrimental to your actual plants).

The better option is manual removal + big water changes immediately after over time. While keeping up feeding and lighting to help your plants be healthy. While you will have a lot more algae growth originally, you'll also actually have plant growth. Over time with manual removal of algae and the plants growing better and better, they'll actually start to outcompete the algae for nutrients. That's the only real solution to algae long term. Oh and an algae turf scrubber too, that works really well too. But that takes up space.