Do I have to flush coco or soil, and how do you do it?

Flushing just means watering with plain, pH-adjusted water until the run-off comes out close to what went in. Coco is the one medium where salts creep up fast, so the habit that works is letting roughly 15–30% run out at each watering. Soil is more forgiving. Flush it if the run-off EC is far above your input EC, or if you see burnt tips and lockout.

Whether a flush in the last 1–2 weeks improves the finished product is debated, and the evidence is thin, but many brand charts still end with plain water. Check run-off with a pH/EC meter. A flush product like CANNA Flush goes into the irrigation water; follow the label, mostly used in the last 1–2 weeks.