Just good ol ph-ed Tap water, start two weeks from harvest, or where you think your plants will be ready two weeks out, flush too soon and you’ll lose potential bud gain. Run water over your soil till they start watering from the bottom. Continue to add a good 1-2 litres . Let it sit 20 mins and continue the process. Do this till your run off water is sitting around 50ppm . If you don’t have one of those fancy devices just do it untill your run off is running clear. Continue to water your plants with only ph-Ed water for the remainder of grow. It’s better to let the soil/medium dry out as well two/three days before actuall harvest. You can purchase some flushing products for “smoother/flavour able” smoke but I’ve never used before. All this information I’ve given I’ve gained threw other growing forums and threads on the web. Cheers !!