mikebarg

Here is a picture of what I believe is LOBELIA CARDINALIS. It's really a nice looking plant. It's growing with only the roots submerged.

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DaFishMan

Cardinal Flower often even grows near the banks too, not just in the water.. That plant should adapt to your tank. I have 2 plants of it in my tank, quite smaller leaved then your pic but a similar structure. Try for a smaller submerged plant which should adapt a little easier.. (but hey ! why not have some emmersed in pots too ?)

Cavan Allen

I think you're right and it is Lobelia cardinalis.