When you have a new tank it is full of silicates (mostly from new sandbeds). Diatoms feed off of those silicates, once they use them all up the diatoms will go away on their own. Or you can add certain snails and hermits that eat them as well.
You will get diatoms even with RODI water if your tank is new. If you are just starting out its a natural part of the cycle and will go away on its own. If you are using tap water it might be tougher to get rid of permanently.