Salvinia roots only grow to about 1 to 1.5 inches long no longer. Salvinia also gets CO2 from the air and it doesn't need much light to do well. Salvinia like all other plants prefer Annonia but will if necessary use nitrate. Even with just nitrate it grows very well.Plants cannot use N2 from the air.Salvinia does well with high humidity and high light or water spray and a lot of water movement doesn't hurt it.
Your tank simply ran out of nutrients nutrients plants need are nitrogen potassium, Calcium , Magnesium, Phosphate, sulfur, chloride, iron, Manganese, boron, Zinc, Copper, molybdenum, and nickel.