A dialysis system includes a fresh dialysis fluid pump, a used dialysis fluid pump, a fresh fluid flow path, a used fluid flow path, and a dialysis fluid cassette carrying a balance chamber, the balance chamber including a first rigid portion, a second rigid portion, and a flexible sheet located between the first and second rigid portions, the flexible sheet dividing the balance chamber into (i) a fresh dialysis fluid compartment connected fluidly to the fresh dialysis fluid pump via the fresh fluid flow path, wherein fresh dialysis fluid contacts the first rigid portion and a first side of the flexible sheet, and (ii) a used dialysis fluid compartment connected fluidly to the used dialysis fluid pump via the used fluid flow path, wherein used dialysis fluid contacts the second rigid portion and a second side of the flexible sheet.