Nutrients for Orchids Growing in Water by: The Hydroponics Guy
First of all, for the life of me, I don't know how orchids grow in FWC.
Orchid roots need air as much as they need water to grow. In fact, in nature many orchids grow on rocks and tress with their roots exposed to the elements. How do you mimic this growing in a glass of water?
Having said that, your choice of nutrients is critical because there is no buffer between what you put in the water and what the plant gets.
I recommend Dyna-Gro products because they're in a form the plant can absorb right out of the bottle and they also contain all the micro nutrients (https://www.easygrohydro-store.com/nutrition-for-orchids-1/).