olgakurt - What you are seeing in your tank is Silicon/Calcium chemistry. You start off with Silica sand (Silicon Dioxide) in your tank, and add water:
SiO2 + 2(H2O) <==> H4SiO4
Then you add Kalkwasser (Calcium Hydroxide):
Ca(OH)2 <==> (Ca++) + 2(OH-)
which absorbs CO2 from the water:
(Ca++) + 2(OH-) + 2(CO2) <==> (Ca++) + 2(HCO3-)
Now the stage is set for
H4SiO4 + (Ca++) + 2(HCO3-) <==> CaSiO4 + 3(H2O) + 2(CO2)
Notice that all of these equilibria can go both ways, driven to one or the other state as a function of solubility, relative ion concentrations, and (importantly), pH. The solubility of Silica is very pH dependent, but in an odd way. At pH=2 Silica has a solubility of 150 mg/liter, which DECREASES with increasing pH until it bottoms out at about 100 mg/liter at pH=8.3. Then, the solubility of Silica starts to INCREASE RAPIDLY with increasing pH until it reaches 1000 mg/liter at about pH=10 and keeps on climbing out of sight.
Note that these solubility numbers are in the ABSENCE of Calcium and Magnesium ions (divalent ions with charge "plus 2"). This is the key to your problem. Normally, since we have plenty of Ca++ and Mg++ floating around - literally -- in our tanks, Silica stays put as solid SiO2 (sand). This is because the solubility of Silica is LOWEST at normal tank pH (pH=8.3) and because the divalent Calcium and Magnesium ions act as precipitation nuclei (much like dust is necessary for raindrops to form in the atmosphere).
However, when you drip Kalk in your tank too fast, faster than it can find CO2 to react with to the Bicarbonate endpoint, you are both decreasing Calcium ion concentration and boosting pH. (This can even happen locally in small volumes of water at the Kalk entry point instead of tank-wide, if you drip Kalk too fast with poor mixing.)
Decreasing your Calcium ion concentration is doubly bad here, because not only is it acting as precipitation nuclei, but it also acts to hold down Silica solubility through the "Common Ion Effect" (Le Chatelier's Principle). All this gives Silicic Acid the opening it needs to dissociates into the Bisilicate anions (H3SiO4-) and (H2SiO4--), which are the biologically available forms that cause Diatom growth.
Note also that even as H4SiO4, Silica is generally not highly ionized at reef tank pH levels. The ionization can be shown as:
H4SiO4 <==> (H3SiO4-) + (H+)
For example, at a pH of 8.5, only 10% of Monosilicic Acid is ionized, and even as the pH reaches 9 to 10, it still is only about 50% ionized.
So, you must be really hiking up your pH as you add Kalk to be seeing bio-available Silica.
To make a long story short if it's not already too late, I'd have to guess that either you are dripping Kalk way too fast and/or you don't have enough CO2 in your tank to make the Kalk go to (Ca++) and 2(HCO3-) as it should. It's spiking your pH, dropping your Ca++ levels, and making the Silicic Acid dissociate to bio-available form, causing Diatom growth. To stop this, drip slower, increase bioload to provide more CO2, or use Acetic Acid (distilled white Vinegar) to provide CO2 for your Kalk. There is along thread going around on using Vinegar in Kalk. (Search on "Kalkwasser" started by llowwell.)
Good luck.
For more info, see also:
http://www.wqa.org/glossary.cfm?gl=1941