With floating plants the issue is almost always a nutrient deficiency. Plant need 14 nutrients to grow. If you are short on just one your plant will look like this and die.
I have seen Slavinia and duckweed survive complexly under water. Dispite what everyone says plants getting wet is not the issue.
floating plants have access to all the CO2 they need and plants can get by with very little light and yet don't die in extremely bright light of direct sunlight. So if it isn't light and CO2 , it has to be a nutrients plants issue..
If so why did did Flourish comprehensive fail to resolve the issue. Seachem like many other aquarium fertilizer companies assume you are using tap water so they take advantage of that by not supplying nutrient that are common in tap water. magnesium, calcium, zinc, copper, and chloride are often not in the fertilizer or are only present at very low levels that are insufficient for good plant growth. Also many fertilizers assume you overstock your tank. So the fish waist might be present at a high enough levels to help.
Flourish comprehensive has less than 1/10th of copper a plant can consume in a week. It also has less than 1/20th of the needed Zinc. If you have zinc plated iron pipes you would have enough zinc but since the 60's Copper has been used. But today new homes probably have plastic pipes.
Tap water also typically has Calcium and magnesium, which are detected by your GH kit. You have enough. But in some places the water will have very little in it. Plant need almost as much Calcium as potassium. And magnesium is needed at levels similar to phosphorous. And yet flourish has levels close to the amount of iron, a trace nutrient.
Now if your lucky your tap water and flourish will work well. But for me my RO water it didn't because, my water had no calcium, magnesium, copper, zinc, and chloride. And everything died in my tan, including duckweed and the plants in the substrate.
YOur submerged plants are probably ding ok due to low light levels. At low light levels they can get by with low nutrient levels. While the floating plants get more light and therefore cannot get by with what is in the water column. If you are using root tabs the nutrients the nutrients might be trapped in the substate with little to no nurtirnets in the water column for the floating plants.
I eventually resolved my issues by making my own fertilizer. But it might be possible with a different fertilizer. Using this
fertilizer calculator. 22.3ml of flourish trace for every 10 gallons of water would get your trace nutrient levels near what I now use Zinc will be a bit higher than I dose and Molybdenum is a little lower than I would like but I think it would work. It all depends on what you have or don't have in your water source. If doesn't have iron so you would have to fertilize with iron to 0.1ppm. You could use Seachem iron but it doesn't last long in the water. So I would break the iron dose to 0.03ppm over 3 days during the week. Do a 50% water change once a week to remove any buildup.
If that doesn't work your might have a phosphate or potassium deficiency.
