This unit is specifically designed for indoor usage
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Hello, I recommend changing the water at least once a day.
You do not need external ventilation for the cooler.
Having had house sized evap coolers, the water is pumped up and dripped down the meduim. I assume that this is the way this one should work too. My problem is that the medium to too thin to provide much cooling. Also, evap coolers do not work well when dew point is above 50-55F regardless of Relative Humidity.
I use mine to cool off my chickens, see no reason why it would not help with the iguana, humid conditions, a little moisture should not effect it.