OK, here is the mystery.
I tried to grow hornwort once in my big tank (2.5 watts per gallon, 78 degrees, adequate filtration). It was semi-floating, but immediately started to turn brown and died. I transfered the one living piece of it into my bettas aquarium (5 gallons, unfiltered, 80 degrees, no light except indirect from window). It grew large and turned a deep green. Nitrates 10ppm in betta tank, 25 ppm in big tank, no nitrites. I fertilize the big tank once a week. Why did the hornwort grow in the betta tank and not the big tank?