A partial water change only removes some of the water in the tank, so the figure you need depends on the water that is actually in there once substrate and decor are taken out. Enter the total volume, the share taken up by substrate and decor, the change share and the interval, and this tool shows how much to drain each time and the total across 30 days.
The math is actual water = total volume x (1 - substrate share) and volume per change = actual water x change share, and the formula used is printed at the bottom of the results. There is no governing body that publishes a recommended change share or interval for ornamental fish, because both depend on stocking density, filtration and your own water test readings. So this tool asks you to enter the figures from the reference you are already using (as of October 2026).
This tool does not decide a correct change share or interval for you, and it does not judge whether a change is needed from ammonia or nitrite readings. The 30-day count is rounded down, so a real month may fit one more change. This tool is for reference only and does not replace veterinary care. Contact your veterinarian or an aquatic animal hospital right away if your fish gasp at the surface, scrape against decor or show other symptoms.
Frequently Asked Questions
There is no officially recommended figure. Enter the share you settled on from your own reference or water test results. This tool only converts that share into a volume.
Enter 0 and the full tank volume is treated as water. Tanks with deep substrate and large driftwood hold noticeably less water than their rated volume.
No. The interval is an input, and this tool does not judge it. It only works out how many changes fit in 30 days and the total volume.