Product review (3 stars): Try dish detergent and vinegar
I've owned Venta Airwashers in each room for a decade. This season the water treatment was out of stock when I needed it. After a comprehensive and frustrating web search, I settled on a few squirts of (hand) dish detergent and a quarter cup of white vinegar, in place of regular use of Venta water treatment and annual use of Venta cleaner. So far, I have found this to be a superior alternative, regardless of cost.

Venta offers various rationales for their additives, to take our minds off the possibility that we've bought into a razor-and-blades, spoon-and-coke business model. The water needs to form a film to properly evaporate; a bit of soap takes care of that.

Sediment build-up? I've always had to take an old toothbrush to the "gear" portion of the spinning disk arrays, to clean off brown gunk each time I change the water. I live in a dirty city with complicated water; your results may vary, but a purpose of the Venta Airwasher is to trap gunk in the air itself; this need for cleaning is proof that the airwasher is working. I always used to dread this step (far more than paying for additives). The brown gunk comes off far more easily, with my homemade solution, and I see no need to ever buy Venta cleaner again. Ideally, any water treatment should double as a cleaner, removing the need for a separate cleaning product. Mine passes this test; their's does not.

Also, the water needs to not go "off" between changes. In principle, dish soap alone can't be counted on here; ever smell a wet sponge that went too long in kitchen service? On the other hand, that sponge is exposed to food, while there is no natural food source in the Venta water. Nevertheless, one might have to discard the Venta Airwasher after one incident of smelling like a forgotten gym bag, so I proceed with caution. pH follows a logarithmic scale, like light or earthquakes, and it takes very little to make water too acidic for most life forms. I can barely smell a quarter cup of white vinegar, and only at first. The vinegar is likely to be playing a crucial role in the "self cleaning" nature of my solution, and I see no signs of damage from the acidity, so I continue to use it.

Also, my units are old enough to develop a "squeak" once per revolution when the water gets low. I don't like waking up to this. I've heard no squeak since changing to this homebrew solution. I have no idea why, but I'll take it.

So I'm entirely happy with this homebrew solution. One should be wary of placebo effects using a Venta Airwasher: I have expensive humidity indicators, and I need to block exhausts in my apartment, and perhaps kickstart with a pot of boiling water, to get measurable humidities from a large unit every 400 square feet. At best, I see 30% humidity, which is nevertheless much healthier than 15% humidity. I have friends in same scale apartments relying on smaller Venta Airwashers, and I don't need to burst this bubble for them, but the placebo effect is a real concern here. So how am I doing? I need to fill each unit twice a day, good. My units are now easier to clean, good. The water doesn't go off, good. If everything else is a placebo effect, I'll take it.