when do I start to fertilize a thanksgiving cactus and with what chemical
Certified GKH Gardening Expert
Since these are epiphytes, similar to orchids, they will feed a little differently. The fertilizer of choice needs to be highly soluble, and will be easiest if liquid.
I recommend an orchid fertilizer cut to 1/4 strength. Spray this on the pads, and into the soil once a month.
This article will give you more information on the care of these: https://www.gardeningknowhow.com/ornamental/cacti-succulents/thanksgiving-cactus/thanksgiving-cactus-plant.htm