Same exact thing happened to me. I asked if fish were allowed, they said no, and I said I'm not interested in the apartment. They quickly changed their mind and said anything up to 40g on the 3rd floor. I had my 10g at that time and was planning on my 30g long. I built a stand for it, moved it in. I had the maintenance guys add an outlet by the tanks and when they did they said that I can't have ANY fish tanks... I fought back and forth with the landlord and eventually they gave in. I moved out a few months later anyways.
As far as the stand, if you are doing a sump you could make something like my stand for my 30g and 10g, just throw some doors on it. I'll provide the link to my build thread, the link for the plans is in my post. The stand is good without any center supports up to a 72" span. I used 2x6's at the bottom to raise my 10g tank up a little higher (the plan calls for 2x4's). I finished with some 1/4" birch and some rustoleum brand gloss paint.
30g long and 10g stand