Salvinia is a sun worshipper and a fert hog..
definitively needs sufficient light to thrive.. It'll do in lower light conditions but than it will stay very small with inhibited root development. Its not the easiest floater around and not the best choice for a duckweed index depending on your light source. I've been growing it for several years different kinds, the natans and the cuculata, both have simular needs.
Since the tank i'm growing it in at the time is naturaly lit with some additional 7 watt low led lights during the winter periode. And i realy see the impact of the winter periods low light intensity on this plant. I keep the same fert schedule all year round. In the summer it grows bigger with longer and beter develop roots. In te winter older growth dies off new young growth stays significantly smaller with less developed roots.
This is what you can expect from Salvinia under low light conditions, see the browning, i only have this during the winter.. The Hygroriza also is impacted by the lower light intensity in the winter but strugling less than the Salvinia. Could be a combination with temperatur since it is an open top tank and keep it at 22°C, in the summer it naturaly is a few degrees above this. This tank goes through a seasonal cycle regarding temp and light.
Duckweed is an indestructable evergreen bugger this stays green all year round even outdoors.
So not all floaters are ideal duckweed indexer if it's tropical and you don't know what to expect how it reacts to other parameters than ferts only.. In my case i definitively know it's not ferts causing this because i see it improve when light and temp goes up.
Here you see the poor root growth in low light condition at 22°C..
High light a tad warmer..