I would recommend that you follow some of the GREAT advice given to you here by members here, and forget the advice given by a couple of people. I definitely would not go about arguing with the Land Lord....... That is a setup for total failure for all future relations with him.
We hobbyist run risks with from floods to fire. What ever his fear, or concerns are he is rightfully entitled to them. You need to do as others have already advised: sit down with him discuss what his concerns are, and how can you show him that there is no need for concern. Such as having someone he (landlord) trust to come look at the structure beneath the area the tank is sitting to prove to him that there is nothing to worry about, or to show you that there is an issue.
Also, if it is the extra water, or electricity being used by the addition of the tank you need to come to an agreement as to what is a fair amount of extra cash per month that works for you both.
If his fear is that of floods, then you have to figure out a way of containing any water from hitting the floor, and going down to the floor below (be it basement or not). This one is not so easy as it would entail you emptying the tank, constructing some sort of catch basin to sit the tank in. Trust me my tanks have had many flood through out the years for one reason or another. I'm grateful that in every event I have been in my own house, and not renting.
One tank (brand new) sprung a leak in the middle of the nigh at the bottom seam, besides flooding my dining room floor it leaked down in to the basement as well.
Another leak was caused by a hose coming loose while I was at work. Another one was from the hose to the ATO coming out of the sump. Another from the hang on skimmer leaking. These have happened at different times through out the 40+ years that I have had aquariums fresh as well as salt.
Anything can happen...