Water Change Calculator

How the dilution math works

A partial water change is a straightforward dilution: whatever percentage of tank water you remove and replace, the remaining old water's contribution is scaled down by that same percentage, and the new tap water's own reading (usually zero for nitrate, sometimes nonzero for other parameters depending on your source water) is added back in at that same percentage.

Why one big water change won't always fix a high reading

A single 50% water change only removes half of whatever's dissolved in the water — it doesn't remove all of it. Bringing a very high reading down to a comfortable target sometimes takes either one larger change or two smaller changes spaced a day or two apart, rather than a single change of any size.

Why consistent water changes matter more than the exact number

A steady weekly or biweekly water-change routine keeps nitrate and other dissolved compounds from accumulating in the first place, which matters more for long-term water quality than getting any single change's percentage exactly right.

Source: The dilution formula is standard mass-balance chemistry (the same C₁V₁ logic used for any solution dilution), applied to tank volume and parameter concentration.