@plantbrain: just checking, but 5 times the recommended excel dosage for a 55 gal?
Thanks all! I'll increase Excel and reduce photoperiod to 8 hours.
I cannot control single tubes, so I may look at the screen approach.
FWIW, I have (2) 54w T5s on a non-CO2 40-gal-long, and I don't have the option to use only one bulb either, so I use the screen approach and run them 10 hours a day on a split photo period (5 hours on, 5 hours off, 5 hours on). Seems to work -- everything is growing well, some things better than others. Water sprite and brazilian pennywort growing like weeds. Red myrio has been trimmed trimmed/thinned several times in 2 months. Swords are all growing new leaves. Banana plant went from 2-3 leaves to 7-8 huge leaves in a month. Bacopa carolinia is sprouting new plants at the base. Vals are growing slowly. Microswords are the only thing that are struggling.