I have a 5 gallon terrarium where I keep all my plants. I keep about two inches of distilled water in the bottom at all times. The water is getting to be pretty dirty looking, and I want to know if it's bad that the water just sits there all the time. Will this encourage fungus and mold that will harm my plants? Should I let it dry up every do often to kill of anything living in there, and then fill it back up again? Thanks for all your answers in advance.