A Japanese study found tap water silicon (Si) as a key component of insoluble particles in online dialysis fluid. Regional tap water Si variations affected dialysis water Si (0.4-0.7 mg/L). Commercial fluid had lower Si (0.1 mg/L) and no Si particles. The study recommends considering local tap water Si when designing dialysis systems to achieve ≤0.1 mg/L Si in dialysis water.