None of the low-flow toilets are designed to empty the tank...they use the water height in the tank to give the amount they do use more oomph. So, the modification you made changed the toilet into a non-conforming 3+ gallon flush tank. There are quality toilets out there that do work fine with the lower flows. So, you are not getting the water savings you paid for. Pay a little more now, or pay forever for more water and sewer charges; your choice.