What do you put in humidifiers to keep the bacteria down?

The stores that sell humidifiers also sell chemicals to use in them. There is a
bactericide sold for the purpose. Read the label carefully to make sure it is
for your type humidifier. If you can't find it, use a few drops of pool
algicide, which is the same thing only more concentrated. The newer model wick
filters now have a bactericide built into them, but I use the chemical anyway.

Iris,
Central NY, Zone 5a, Sunset Zone 40
"The trouble with people is not that they don't know but that they know so much
that ain't so."
Josh Billings (Henry Wheeler Shaw), 1818-1885