Yeah, when I had frogbit in my nano planted tank with just shrimp, the roots grew to over 12" in length and went all over the place, it was crazy! I'm guessing that's just how it grows and it's not a lack of nutrients or anything, I had ADA aquasoil in that tank and used tetra plantamin fertiliser.
I had some rubbish
Salvinia auriculata from plantsalive and it just rotted and died like yours sounds to be doing, it was also being eaten by something that I later discovered to be moth larvae when 3 small moths flew out of my aquarium hood upon opening one day
.
Here's a picture I took of one of them (for size comparison, the big circular object on the upper left is a thumbtack).
The thing with
Salvinia auriculata is that it appears to grow in different forms depending on the enviroment. In ponds or when it receives direct sunlight, it tends to grow
like this (sometimes bunched up even more), and when growing in the relativity low light of an aquarium it usually looks
like this.
For me anyway, whenever I've put the high light form in an aquarium, it just died. Granted one time it was being eaten as well though, so I cannot say for sure lack of light is a problem.