Red root floaters dying

So I am trying live plants for the first time. I've only cared for a moss ball before this and they're about as easy as they come. I decided I wanted to try floating plants to help with nitrates and my LFS only had red root floaters, so I decided to give them a go. They have been in a quarantine tank for about 1.5 weeks. The tank is about 2.5 gallons. I haven't tested parameters in the quarantine tank but guessing by my tank and tap tests the parameters should be 0 ammonia, 0 nitrite, 30 nitrate, and about 8.2 pH. About half of the floaters are not looking good. There's weird spiderweb looking stuff on the surface of the water (string algae? It doesn't appear green though.) They are turning brown and the leaves are going transparent and thin. I know this can happen from too much flow, but the quarantine tank isn't filtered because it's just the plants.
About 1 week into the quarantine, I dosed with Flourish for the first time. I waited to see if they'd do ok with just my high tap nitrates but they weren't consuming them, so I got Flourish. Did I maybe just deprive them of nutrients too long? I've moved the few plants that still look ok into a tiny tank to finish their quarantine. I'm planning on adding them into a plant ring in my main tank in about 3 days. Is this still ok?
When I got the plants I did a light hydrogen peroxide dip, but they seemed completely fine afterwards and looked ok for a week before any of this started happening.
Any help is appreciated!

RRF like a lot of light. What lighting do you have? Also, mine benefit from iron boosts, like API Leaf Zone. You may want to try putting them by a sunny window and see if that helps. Do you have any pictures?

eatyourpeas said:

RRF like a lot of light. What lighting do you have? Also, mine benefit from iron boosts, like API Leaf Zone. You may want to try putting them by a sunny window and see if that helps. Do you have any pictures?

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Yes, here's a photo of one of the worse off ones and some of the weird string algae looking stuff. Since they're in a temporary setup they are getting indirect light from windows because I thought I read that they shouldn't get hot from lighting, but I'm going to try more direct lighting with the living ones.

I get the white stuff when they are light deprived, specially in the algae tank that I breed the ostracods in. Put a couple of the RRF in a little plastic cup on a window sill where they can get direct sunlight and see if they show improvement. Their roots are brown which is not a good sign, but they can bounce back, I think.

Also, stop the ferts for now and do not put them in the big tank yet.