You want to find load bearing walls and distribute the load across the supporting joist connected to that wall. This will carry the load all the way down to the foundation. Now that was the easy part, the hard part is just how much more weight can they carry when they are designed to carry the weight it has now. You truly would need a structural engineer to do the load calcs. and determine what you could safely support or what you would need to do to increase the load capability. Now all this said, larger tank by what means is larger, 125 to a 150, 250 etc? lol
So to answer your question in the short, there are numerous things to consider so in general isn't a good answer to your question. Sorry, that isn't what you wanted to hear I know but food for thought.