I ended up removing the sand bed from my 180 gallon reef tank after battling Amphidinium Dinoflagellate for a couple of months. The nitrate (7 ppm) and phosphate (0.07) levels have stabilized, so I am thinking about adding new sand back to the tank. I am wondering how long people in a similar situation have waited before returning their sand and if you experienced a return of the dinoflagellate after returning the sand.