Diatoms tend to come and go as they please. Is this a new tank? If so, it's all part of the normal process. new tanks are high in silicates from the fresh silicone used to seal the tank edges. Diatoms thrive in silicate-rich environments. If all part of "New Tank Syndrome" to let them consume the silicates.
You may also have silicates in your source water. Some believe that running phosban or GFO
will help to remove the silicates from the water, but I haven't been able to prove this yet.