The landlady will be living in another room in the same apartment so she'll know they're there. I doubt she'll have a problem with it whether I get the terms changed or not, but...
I've dealt with a few nasty people in the past that will used any means they have to get you. I'd rather not move in to a place and already be violating a signed agreement that could later be held against me, ya know? If I absolutely couldn't move my tanks in, I'd rather just find another place to live and have the peace of mind that I won't have any legal/contractual surprises. I'll also be visiting the closest dive shop to see if they know about any city ordinances or the like that might be the reason for this.
So back to my original question: any ideas of how to convince a non-diver of the lack of danger involved in residential SCUBA tank storage?