I was an exterminator while I finished up last 2-years of college. If it is a roach or ant treatment, just cover the tank with a damp towel as mentioned above and unplug the pump. That was the advice we gave... however now being a fish hobbyist I think it may be better to leave the pump on with carbon filtration running. I don't know, I could be wrong about that, but definitely cover it with a damp towel. If the house is being fumigated for termites, I don't know what you need to do.