Product review (5 stars): This Is What It Is Supposed to Be
Nothing super-fancy to say about it.  This is the real filter for the Venta-Sonic 200-series models of humidifiers.  It easily screws right on to the upper side of the reservoir's gasketted "plug" (for lack of a better word) which in turn screws into the reservoir's fill opening.  It prevents the formation of that very fine white dust that many ultrasonic humidifiers put out if you DON'T use a filter.  The nano-silver product prevents that gross mold from growing.  You still need to clean the humidifier (the base more so than the reservoir), but since you don't have the mold growth, cleaning is easy.  One filter lasts an entire heating season for me.

The only problem I have is that they have apparently quit making the 207 model, which has the LED display panel, which I found very helpful.  But for the model 205 (and for those who still have the model 207), this is the filter, and it is great.