Frogbit Problems

Hello! I'm doing my first planted tank, and I'm having problems with my frogbit. Many of the leaves / stems have turned brown and rotted off of the main plant. Also the roots don't seem to be growing. At first I had the roots planted but I thought the frogbit leaves were supposed to be at the surface so I uprooted them. The roots also look a little rotted. I'm attaching pictures of the whole tank, the frogbit leaves, and the roots. The frogbit has been in there for ~1 - 2 weeks. Every week I dose the tank with aqueon aquarium plant food, and there are no fish in the tank yet.

Tank info:

size: 20 gallon tall
substrate: fluval bio-stratum
plants: anubias sp, crested java fern, weeping moss, and some other plants I forget the names of
decorations: 2 pieces driftwood, assorted dragon rocks (both from pet stores)
pH: 6
nitrites: 0
nitrates: 10
temp: 76 F

Thanks!

I see no frogbit in the pictures above.
Frogbit is a floating plant, with relatively small leaves. It is not suppossed to be planted; in fact it would be very difficult to succeed at planting it, because the leaves have vesicles filled with air, which keep pulling them to the surface. And the roots are
I imagine what you are calling frogbit is the plant at the surface, with large, ear-shaped leaves. That looks like some sort of Araceae (same family as Pothos, and as Anubias). It has very strong leaves that in shallow water serve as anchors.
Perhaps other can offer additional insight. Good luck.

[It has very strong leaves] I meant roots...

I don't know much about European frogbit. (Looks like the European variety) But I heard they prefer slow-flowing water. What is it like in your tank?

Hydrocharis morsus-ranae

Byron has written a few times about three different species of frogbit. One thing he mentioned is that when he bought the temperate water H. morsus-ranae, which had been mis-labelled as Limnobium laevigatum, it never did well at the tropical temperature of his tank.