Seachem comprehensive has no nitrate in it and it has very little nitrogen in it. When I first used it in my RO water tank my nitrates went to zero, my plants stopped growing and phosphate started to build up. I had to add nitrate to get the plants to grow and reduce my phosphate levels. Flourish comprehensive is designed to be used with tap water. It doesn't work with my RO water so I stopped using it. Seachem comprehensive is depends on your water hainve some nutrients. Since RO water has no nutrients it didn't work in my tank. Hopefully it will work in yours but it may take a month or more to know for sure.
There is no question that plants need nitrogen as well as potassium calcium, magnesium phosphate, sulfur, chlorine, iron, manganese, boron, zinc, copper, Molybdenum, and nickel. If just one of these is missing plant growth will slow or stop and the plant may die. This was proven in a lab before you were born. I prefer to keep my nitrate level as low as possible without going to zero. If it goes to zero I know all other forms of nitrogen have been consumed as well as the nitrate.