I would say it depends as metweezer says on the chemicals. They've fumigated the school several times since I've been keeping tanks here, and the first time they did it I paniced, raced around trying to get everything covered etc ~ didn't lose anything. Second time they did, I was gone, it didn't affect anything. I wouldn't say don't worry about it, but I would say to do some additional checking ~ it would really be the pits to lose everything if what they are spraying truly is toxic to the tank. JME