I'm not sure what you mean by "tank does not fit, what do they put back there to make it fit?" If the toilet doesn't fit, nothing you can put back "there" is going to make a difference.

Toilets comes sized for various rough-ins. A 12" rough-in is most common, but there are toilets made for 10" or 14" rough-ins too. You simply need to measure the distance from the wall to the center of the toilet waste line to determine what toilet rough-in size will work for your location.

Perhaps you're worried about a larger gap than you had before? Is that correct? If so, have a look at the toilet specs because although most are 12" rough in (wall to centre of flange) the distance from the tank to the wall varies. Some are 1/4", some are over 1". Check the specs.

the old toilet is 10 1/2 inches.

i am told the sizes available are 10 and 12 inches.

I did a bathroom reno before and the toilet tank did not fit flat aginst the wall. there was a gap. the new tiles were thinner than the old thicktiles.

What's the worry about a gap? Nearly every toilet I've seen has a gap, except the ones that are hung on the wall as you see in public bathtrooms.

As long as the tank sits well on the bowl and there is no rocking issue, the gap behind the toilet shouldn't matter.

We just bought a toilet - didn't measure the rough-in before we went to the store (oops!) but almost all available were 12". Came home to discover that our old toilet is 14". Does this mean the toilet will fit, but there will be a 2" gap between the tank and the wall? The plumber is coming on Wednesday, and I'll call him Monday, but if there's anyone who can put my mind at ease in the meantime....

I have a toilet that is pretty far from the back wall it came that way from the constrution crew. It is ok, you get use to it. My dad said it was nice cause if you had to work on the toilet you had room to do it.

I believe the skirted Toto models are designed to be adjustable in terms of the distance from the wall (and the plumbing rough-in). Not the unskirted models, just the skirted ones. I can't describe the "mechanism," but they can be fitted for plumbing rough-ins that are anywhere from 10 to 14 inches from the wall, so I assume that's to eliminate the space behind the tank?

(Would love some clarification myself as to how this works on the Totos!)

ct - those TOTO wc's will rough in at 10, 12 or 14" depending on the trapway you get. The standard one that comes with it is for 12" You have to order the 10 or 14" if you need it. They are around $50. They are not infinitely adjustable so won't really help out the OP.

Here is a link that might be useful: toto inst.