Repost your photos in a couple of months. A few days is far too short a period of time to judge plant health.
Commercially grown plants are cultivated “emersed” rather than “submerged”. This way the plants can easily get their carbon requirement from atmospheric CO2
. They can be grown quicker which makes the operation much more commercially viable. You take that plant, put it in your aquarium, cut off its source of CO2
and the plant goes into survival mode. It starts to use up its stored carbon and the leafs melt. You may lose all your original growth to melt but new leafs will have a structure more suited to its new environment and get its carbon from the water. Plant melt is a normal stage in aquarium plant growth. To judge the health of a plant look for new growth rather than what might be happening to the original growth, and judge it over extended periods of time.
Fertiliser doesnt cause algae growth. It might make algae growth worse in conjunction with other factors, but isnt a cause. Many aquarists dose extremely high concentrations of nutrients without it causing algae. Plants will benefit from an all in one fertiliser like flourish, and rooted plants will benefit from root tabs in the absence of a dirt based substrate.
I wouldnt worry about nitrate for now. If down the line your plants look like they are suffering from nitrogen deficiency and your nitrate is suspiciously low then you might want to take action, but as long as you have some nitrate you can be sure your plants arent consuming them faster than your livestock produces them.