AquaPulse Tools · Free web assistant
Water Change Assistant
A few quick questions help this assistant work out how much water to change and give you a plain-language estimate of what it may do — not just a percentage calculator. Runs entirely in your browser — no login, no account, nothing saved or sent anywhere.
Water change assistant
Your results
Answer the questions and press Calculate to see your water-change plan and what it may mean for your tank.
Confirming the estimate against a retest
This plan is an estimate based on the readings and assumptions you entered — real aquariums rarely match a formula exactly. AquaPulse's Water Check can hold today's reading and your post-change retest together, so you can see how close the estimate actually was, and carry that forward into your next planned change.
Water Check works from the readings you enter — it doesn't take automatic readings and isn't a substitute for testing your water with a reliable kit.
About this assistant
What it does
Asks why you're considering a change and a little about your aquarium, then calculates the volume of water to remove and prepare, shown in litres, US gallons and UK gallons.
If you add readings for ammonia, nitrite, nitrate or phosphate, it also estimates what each might look like after the change, and after a second identical change, using simple proportional dilution — and gives contextual next steps rather than a bare number.
How to use it
Work through the four steps, then press Calculate. Choosing a reason in step 1, along with your aquarium volume and planned percentage in step 4, is required; your aquarium context in step 2 and any readings in step 3 are optional and only refine the result.
Don't know your volume? Use the dimensions estimator near the volume field for a rectangular or cube-shaped tank.
Why this is an estimate
The dilution figures assume the replacement water mixes evenly, your entered volume is reasonably accurate, no extra waste is produced or removed in the meantime, your test readings are accurate, and the replacement water genuinely has the concentration you entered. Real aquariums rarely match a formula exactly — retest after changing the water rather than relying on the estimate alone.
Why a percentage isn't automatically a recommendation
This tool calculates whatever percentage you enter — it doesn't judge whether that percentage is right for your tank, and it never labels a result as safe, ideal or recommended. Presets like 25% are common starting points, not a rule. See our water-change guide for how to decide what's sensible for your own aquarium.
Why aquarium context still matters
Stocking, feeding, tank maturity, live plants, source water and whether you keep freshwater or saltwater/reef livestock all affect what a sensible water change looks like. This assistant uses your answers to adjust context and caution — it doesn't know your tank's full history, so use it alongside your own judgement and testing.
Why pH isn't included
pH is measured on a logarithmic scale, so it doesn't dilute in a straight line the way ammonia, nitrite, nitrate or phosphate roughly do. A simple mixing formula would give a misleading answer, so this assistant deliberately leaves pH out. See our guide to aquarium pH for why stability matters more than chasing a number.
Want the full picture?
This assistant handles the arithmetic and context. For how to decide your frequency and percentage in the first place, when to change water early, and what to avoid, read our full aquarium water-change guide. If a specific reading brought you here, our ammonia, nitrite and nitrate guides explain what a result means and what to do about it.
Good to know: This assistant provides general informational estimates only. It is not veterinary advice and does not replace testing your own water or consulting a specialist for serious livestock health issues.
Common questions
Frequently asked questions
Do I need to enter my water test readings to use this tool?
No, readings are optional. You do need to choose a reason in step 1 and enter your aquarium volume and planned percentage in step 4 to get a result. Adding readings for ammonia, nitrite, nitrate or phosphate in step 3 is optional and refines the estimate — the assistant then works out what each might look like after the change, and after a second identical change, using simple proportional dilution.
How accurate is this water change calculator?
It's a genuine estimate, not a guarantee. The dilution figures assume the replacement water mixes evenly, your entered volume is reasonably accurate, no extra waste is produced or removed in the meantime, your test readings are accurate, and the replacement water genuinely has the concentration you entered. Real aquariums rarely match a formula exactly, so retest after changing the water rather than relying on the estimate alone.
Does this tool tell me the right percentage to change?
No — it calculates whatever percentage you enter, but it doesn't judge whether that percentage is right for your tank, and it never labels a result as safe, ideal or recommended. Presets like 25% are common starting points, not a rule. See our aquarium water-change guide for how to decide what's sensible for your own aquarium.
Does this assistant know if a water change is safe for my specific tank?
Not fully. Stocking, feeding, tank maturity, live plants, source water and whether you keep freshwater or saltwater/reef livestock all affect what a sensible water change looks like. This assistant uses your answers to adjust context and caution, but it doesn't know your tank's full history, so use it alongside your own judgement and testing.
Why doesn't this assistant include pH in its calculations?
pH is measured on a logarithmic scale, so it doesn't dilute in a straight line the way ammonia, nitrite, nitrate or phosphate roughly do. A simple mixing formula would give a misleading answer, so this assistant deliberately leaves pH out. See our guide to aquarium pH for why stability matters more than chasing a number.