You'll need to make sure your tank is cycled before dropping in the oto. They are extremely sensitive to uncycled environments (in my experience).
Aren't the diatoms usually present in the final stages of cycling? Seems like waiting for the tank to finish cycling would somewhat eliminate the need for the oto in the first place.
On another note, I added four of them to my two week planted tank and they seem to be loving it... haven't checked my levels recently though.