I use it with pure tap water without any special agent. It is an evaporator. This means that no spores/bacteria/viruses are dispersed in the air as with an atomiser.

This model is based solely on a passive physical principle, i.e. the water in the reservoir rises by capillary action in an alveolar structure, and it is the air current generated by the fan that humidifies the air as it passes through this alveolar structure. So there's no danger to your cats. You don't need to use distilled water, but if your tap water is very hard, the honeycomb structure will clog up with limescale deposits, with a risk of microbial growth. The rule is therefore to maintain the appliance correctly, by changing the structure regularly, if I'm not mistaken about every two months, or by using a disinfectant to prevent bacterial proliferation, with hydrogen peroxide for example (no smell, no air pollution). I hope I've answered your question!