Net Water Volume
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Nominal minus substrate & decor displacement
Aquarium Water Change Calculator
Plan precise water changes for freshwater or saltwater. Get remove/refill volumes, bucket counts, dechlor/remineralizer dose, and optional nitrate/salinity targets. Supports US/metric units and subtracts displacement (substrate & decor) for accurate net volume.
Water Change Summary
Enter your tank, change %, and (optional) targets.
Remove / Refill
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Exact change volume in gal/L and buckets (if set)
Conditioner / Additives
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Dechlor or remineralizer based on new water volume
Salt Mix
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Required salinity of new water & salt weight estimate
Nitrate Result
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After this change; multi-change plan below
Temperature Tip
Match ±1–2°
Pre-heat/cool new water before refill
How This Water Change Calculator Works
Accurate changes use net water volume: we subtract substrate and décor displacement from the rated tank volume.
Change volume = net × (change% ÷ 100). For nitrate, the new concentration after a single change is
NO₃′ = NO₃now × (1 − f) assuming nitrate-free source water, where f is the change fraction.
For salinity, blending follows S′ = Snow(1 − f) + Snewf, so the required new-water salinity is
Snew = (Sgoal − Snow(1 − f)) / f.
Best Practices for Water Changes
- Match temperature within ±1–2° of the aquarium.
- Treat new water volume (not full tank) with conditioner or remineralizer.
- Vacuum substrate lightly; avoid deep gravel cleaning in newly cycled tanks.
- Large corrections (nitrate, salinity, hardness) are safer as several medium changes than one huge change.
- For reef tanks, pre-mix salt 24 hours with heat and circulation; confirm salinity and alkalinity before use.
FAQs
How much should I change each week?
Community freshwater tanks: 25–40% weekly. Heavily stocked or overfed tanks often benefit from the higher end.
Why doesn’t my tank hold the rated gallons?
Glass thickness, air gap, substrate, and hardscape displace water. This calculator estimates and subtracts that displacement.
How many buckets is a 30% change?
Enter your bucket size; we’ll display exact counts. Consider a water-change hose for tanks over ~40 gal / 150 L.
How do I lower nitrate safely?
Use the multi-change planner. For example, two 40% changes reduce nitrate to 36% of the original (0.6 × 0.6).
What salinity should my new water be?
Enter current/target salinity and change%. We’ll compute the correct new-water salinity for a one-step correction.
