Some diatoms are a good thing, some reefers even add some silicate to get some diatoms to grow to feed their snails and such.
If it is blooming fast you are probably adding it through feeding. Feeding and top-off are the common ways it is added.
Here is a snip from an artical that touches on it
Silica
Silica raises two issues. If diatoms are a problem in an established reef aquarium, they may indicate a substantial source of soluble silica, especially tap water. In that case, purifying the tap water will likely solve the problem. In such a situation, testing may not reveal elevated silica levels because the diatoms may use it as quickly as it enters the aquarium.
If diatoms are not a problem, then I suggest that many aquarists should consider dosing soluble silica. Why would I recommend dosing silica? Largely because creatures in our aquaria use it, the concentrations in many aquaria are below natural levels, and consequently the sponges, mollusks, and diatoms living in these aquaria may not be getting enough silica to thrive.
I suggest dosing sodium silicate solution, as it is a readily soluble form of silica. I dose a bulk grade of sodium silicate solution (water glass), which is very inexpensive. You may find "water glass' in stores because consumers use it for such activities as preserving eggs. Finding chemicals to buy can be difficult for many people, however, and this linked hobby chemistry store sells to individuals. Ten dollars plus shipping buys enough to last for 150 years of dosing a 100-gallon aquarium, so cost is not an issue.
Based on my dosing experience, aquarists are probably safe dosing to 1 ppm SiO2 once every 1-2 weeks. This is based on the fact that my aquarium uses that much in less than four days without any sort of "bad" reaction. Of course, there's nothing wrong with starting at a tenth of that dosage and gradually ramping it up. If you do get too many diatoms, just back off on the dosing. I presume that all of the SiO2 I have added to my aquarium has been used by various organisms (sponges, diatoms, etc), but perhaps I have more sponges than other aquarists. Consequently, diatoms may be more of a concern in some aquaria than in mine.
I would also advise occasionally measuring the soluble silica concentration in the water, in case the demand in your aquarium is substantially less than mine. If the concentration started to rise above 3 ppm SiO2, even in the absence of diatoms, I would probably reduce the dosing rate because that is close to the maximum concentration that surface seawater ever contains. Additional details on dosing amounts and methods are described in this previous article.
http://www.reefkeeping.com/issues/2004-05/rhf/index.php
and heres one more
http://www.advancedaquarist.com/2005/5/tips
HTH
Man I wish FEDEX would hurry I gotta go to work and need time to acclimate my new corals!
