So they are on the floorboards not directly above the joists, like this?
leg
_________||__________ floor
...||...................||
..joist.................joist
Looking at it as a cross-section through your floor. If so do you know how thick your floorboards are?
It might look solid now, but there's a couple of long-term problems I'd be worried about. The main one being uneven sag over time, putting uneven pressure on the legs and the bottom of the tank. I doubt the floor will collapse and you'll come home to find your tank in the basement, but if you've got six feet putting over 100kg of pressure on pieces of timber less than 2" thick then I'm certain there will be sag/flex of some degree. I don't know if it will be enough to unevenly distribute the pressure enough to crack the tank, but I wouldn't want to take the risk myself.
Not trying to rain on your parade.....