Cover any exposed water with saran (even on the filter) and turn off any bubblers or other air-sucking pumps (be sure to lift higher than the water if you don't have a check valve) and you should be okay. If you still worry, throw a tarp or beach towel over the whole tank. I don't know if "green" cleaners are any better. I suspect they are more likely to be water-soluble and thus more likely to get in the tank undetected (an oily film on the surface will be visible).
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