Hard water is just water with more calcium and magnesium dissolved in it, and those minerals make it harder for detergent to dissolve and rinse clean. The practical fix is to use a little more detergent than the box recommends and stick to a cold, gentle cycle rather than hot water, since heat only helps minerals bond to the fibers. This matters more for muslin than for denser cotton, because the open weave can trap residue if it is not rinsed well; our guide on how to wash muslin baby clothes to keep them soft covers the wash routine we use for Epic pieces in more detail.
Skip the fabric softener. Hard water already leaves clothes feeling a bit stiffer than they would in soft water, and softener just adds a coating on top of mineral residue, which can dull the fabric and clog the weave over time. Tumble dry on low heat or hang the clothes to dry instead; both are gentler on the fibers than high heat, and hanging in particular lets muslin keep its texture wash after wash. If you are not sure which drying method suits your machine, our page on tumble drying muslin baby clothes walks through both options.
Over many washes, hard water minerals can build up in fabric and leave a faint gray or yellow cast, especially on pieces folded away for a while between sizes. A splash of white vinegar in the rinse cycle every few washes helps cut that residue without adding another product to the mix. If staining has already set in on stored pieces, our guide to removing yellow stains from stored baby clothes has the steps. Epic's muslin pieces are double-layer muslin cotton and come pre-washed, so they already start soft and hold up well to this kind of routine care.