Say what? I've had a nearly two inch thick layer of duckweed in my 65 before...
But, I can't keep alternathera reineckii, spell check please... Lol
I couldn't even rescue it by transitioning to emersed growth... Straight melted away...
Iron, iron, iron! That's what I've noticed A reineckii really seems to need to fill out and look gorgeous (see pictures)
I had no problems growing it beautifully when I first started getting my tank planted. At the time I was using the full line of Seachem products, dosing by their instructions and schedule - which has a fair amount of iron, with very little CO2 (was DIY with almost no circulation).
Since I've switched over to EI dosing with powdered ferts, cranked up my CO2 (and provided adequate circulation) and have relied solely on CSM+B for my Iron (I bought Ferrous Gluconate before reading more into iron and have a giant GDA outbreak every time I add some), they just don't grow nearly as well.
My personal headache plant is HC. It seems like no matter what I do, if they grow at all, they grow upward then just break apart
Another pain of mine is DHG. It doesn't die....it just doesn't do anything at all. I had it grow and send out runners ONCE when I made a mistake and flooded my tank with way more CO2 than I intended - which of course the fish didn't like at all