Keeping a plant humidifier from turning slimy, and what the slime actually is

Run one for a season and it goes green or develops a film. That is biofilm and algae, and it is preventable with a routine rather than a product.

What causes it
Standing water, light and minerals. A tank on a sunny windowsill with tap water is close to ideal growing conditions. The white dust on your leaves is a separate problem from the same source: minerals from tap water, atomised and settled.

What actually works

• Empty and dry it daily. The single most effective habit. Biofilm needs continuous water; a tank that is dry for a few hours each day never establishes it. Emptying is more important than cleaning.
• Move it out of direct light, or use an opaque tank. Algae specifically needs light and this removes it as an option.
• Distilled or filtered water for ultrasonic models. Fixes the white dust and slows the film, since minerals are part of what the biofilm attaches to.
• A weekly clean with white vinegar. Fill, sit for an hour, scrub the corners with an old toothbrush, rinse well. Vinegar handles both the mineral scale and the film.
• Never top up a partly full tank. Refilling onto old water is the fastest way to keep a colony alive. Empty, rinse, refill.

What to avoid
Bleach in an ultrasonic tank, since residue gets atomised into the room. Tablets sold to keep tanks clean, for the same reason. And essential oils in anything not designed for them, which degrade the plastic and the transducer.

The unglamorous truth: a humidifier that is emptied and dried every morning almost never needs deep cleaning, and one that is topped up for a week needs it constantly.