Review on 4-Pack Universal Humidifier Tank Cleaner for Popular Drop, Droplet, Adorable, Warm & Cool Mist Humidifiers and Fish Tank by Liz Taylor

I bought these to keep my humidifier clean. They're cute and easy to use, but I have concerns. After dunking one of these in water, I noticed a dark film on the outside of the humidifier that was never there when I used the machine without this product. I'm keeping an eye on it but it's got me a little nervous as I don't know what it is and if I'm breathing unsafe air from using a product that should make me feel safer.