I usually flush on nutrient days. But instead of nutes, they get a good flush with properly ph'd water. Allow the soil to dry for a day or two, then resume your normal nute schedule.

If going with the aquarium test kits, get the freshwater one. The ph range is too high with the saltwater one, and if I remember correctly, were a weird shade of purple I couldn't read well. ( I used to keep a few coral reef tanks, too) I've used those soil test kits, but they are a pain in the ass. You'll want to be testing the water, anyway. You can take a water sample to the aquarium store, and let them test it. Should give you a good idea on what to purchase to raise or lower the ph. to within acceptable limits.

I was curious about the lights being too far, but that sounds close enough, lol.