If you are doing MTS, then yes, and a bunch of times. If you are just using potting soil, then no. I have done potting soil a few times and if I did it again, I would likely let it soak in water, in the tank for a few days, given I had the time. Then you won't get a bunch of air bubbles under the cap. Soil may come up with the bubbles.I've heard different answers so i am quite confused. I plan on washing my sand cap though.