I'm new to keeping aquatic plants, I bought 4 plants at petsmart, one medium Anubis Nana, one small Anubis Nana, one small Anubis Frazeri and one Java fern.  
I didn't know it at the time but the small Anubis nana was dying when I got it, it fell apart immediately, had yellow leaves and a brown mushy rhizome. Anubis rot killed it.

I planted them all together in a new tank and tried to remove as much of the sick plant as I could but I'm still finding long brown pieces of rot hiding in the gravel. I went back to petsmart to exchange the plant for an alive one since it didn't last two days. I replaced it with an Anubis Barteri.

Because the Anubis rot is in the tank I'm worried about all the remaining alive plants, specifically the Frazeri. It had rot surrounding its rhizome but the core was still hard. I scraped as much as possible off but I don't know if I should cut the effected rhizome since the rhizome on that plant is so small already. The pictures show how the Frazeri looks now.

All the plants have brown streaks or parts of the rhizome, but they're not squishy like the other rot. I don't know if this means a death sentence or not. Can they survive the brown streaks?

Isolating the plants won't do anything now   
because the bacteria is in the water and I think it's gotten to every plant.

What should I do?

TLDR: new aquatic plant keeper needs help dealing with Anubis rot