TammyLiz
Fish Herder
I have hornwort in several different environments. It does best in my overstocked low light tank. Second best is in the betta bowls. It doesn't do too well in my slightly understocked, moderately planted almost medium light tank. Older sections of it turn brown and I remove them before they start to fall apart. I speculate it is because hornwort is a really hungry plant and the other plants take up most of the waste in that tank. I don't dose anything. I suspect your tiny guppies aren't putting off enough waste to feed your hornwort.
Tammy
Tammy