Salvinia's roots don't get too long, about 1-1.5 inches is the max. They're pretty low maintenance when it comes to light, and they can even get some CO2 from the air. Like most plants, they prefer ammonia but can make do with nitrate if that's all that's available. And yeah, they can grow well even with just nitrate.
Your tank probably just ran out of nutrients. I mean, plants need a lot of stuff to survive - nitrogen, potassium, calcium, magnesium, phosphate, sulfur, chloride, iron, manganese, boron, zinc, copper, molybdenum, and nickel. High humidity and some water movement don't hurt either.
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