It's not only the volume of water, but how fast it is delivered. If you have a small mirror, take a look under the rim at the holes there to see if they are all clogged up and get back. There are ways to potentially clean them out, but sometimes, it is just time to bite the bullet. I bought my mother an ADA height Toto Drake over Christmas...seems to work great and wasn't too expensive - I think it was around $225, but don't quote me on that. There are a bunch of ADA height toilets around, but 18" is on the high side from what I remember (not much!). Typical ADA height is around 2" taller than "normal" height toilets, but varies. There is a height range that is specified to qualify for that ADA qualification - not sure what it is.