Do I need to reinforce the floor?

[Myka]

Originally Posted by Gripenfelter

I guess I should mention I have granite and marble tile flooring.

So the joists are about 12" in height.

On top of that there is plywood. On top of plywood there is a few millimetres of concrete, then a steel wire mesh, 2-3 mm more concrete, then a 12-14 mm thick granite or marble tile.

Basement is concrete. Floor is a floating concrete pad. I was told not to put a telepost there because it would shatter the concrete where it stood.

I can jump up and down all I want around my 120 tank and barely a ripple in the water. I had throught about joining the bottom of the joists together with 2x4s. Hmm.

You're going to crack the tiles and/or dislodge them or the grout if you put that big tank on there without a lot of reinforcement.

Whoever told you that you will crack the concrete below was probably right (it won't "shatter" though). Do you know how thick it is? You would probably have to cut out a similar shape to your tank (except bigger each direction), and dig out the dirt below to pour a thick footing to put the jack posts on. Depends how badly you want to put the tank there I suppose!