i believe it s over the i beam. honestly man, take off all the dirt already. if it keeps getting worse which it is, that means that floor is going to bust. imagine having live fish and corals it may take a month or a year but it will bust. i would reccomend either crossed beams (3-4) or just get 2 tanks, even with 2 tanks i would still add atleast another beam. why risk it that bad mess up your house and your tanks, plus safety too. when it falls it will also pull your walls, dont think it is just gonna crack and fall, no, because that beam wont crack, it will tilt.
so you proved the floor wont hack it, so why not get on the sane side, and add only 2 tanks, smaller ones too...your forgetting the wight of live rock, equipment, stands, and the actuall glass aquariums etc. its not just weightless tanks with water.
but i mean if you really want wait a week so we can see the pics of the damage..id never do anything that would damage my own house, i rather have a nano if it came to that.