How long has your tank been up? How long have you had the problem, diatoms are common in newer tanks and generally run in cycles and can go away on their own with good maintance, but if you are having a chronic problem then there must be other sources feeding the diatoms.
If you are sure your water source is clean of phosphate and silicates aka RO/DI water then chances are your sand bed or if the sand bed is new your rock is leaching them into the water and it will take some time and patience to basically "cook" the rock in your own tank. If you start over you will most likely end up with similar problems unless you find clean or you cook the rock yourself.
Did you use live rock or dead rock to start?
If you have corals and fish in the tank now then I would recomend running GFO, carbon and doing water changes, probaby 5 gallons every few days or so at least once a week and I would say more. It may take a few months but you will eventally leach out the rocks.
You can take a piece or two out and replace it with fresh rock as well, but if its dead rock I would give it an acid bath and pre cook it.
This new tank process can take months or much much longer depdning on many factors such as how "dirty" the rock is, biolad, feedings and how dilligent you are with the water changes.
In my opinion there is no quick fix for these types of early tank problems. But one thing is when you beat it and the tank finally looks clean, its a great fealing lol.