W3ell, if this helps, I've had mine in my tank (20gal long) for over a year and mine has not only doubled in size but is looking really comical now because it's carrying so many babies. Temp in the tank is 78-79 degrees, I use half a teaspoon aquarium salt to a gal of water, and have had extremely heavy fish loads (not at the moment) yet my Marimo has flourished and thrived (thriven?

). I fertilize when I remember to - it's not a discipline (unfortunately). Probably why I have to replace plants the way I do.
My Marimo is an extremely dark bright green and is just very happy. Sometimes I put it in the airbubble flow and let itrotate and twirl in the bubbles. It's always kinda funny to see my fish all hovered over to one side of my tank, all of them watching this rather large green thing slowly spinning in 'their' bubbles.