Aquarium Water Change Calculator
Compute exact swap volume, dechlorinator dose by capful (Seachem Prime, API Stress Coat, Tetra AquaSafe), and weekly frequency by profile — community, planted, discus, African cichlid, reef SPS/LPS, or shrimp. The split-tank diagram shows old vs new water, temperature-match step, and nitrate dilution math. Calibrated against Seachem and API label rates plus Tom Barr's EI water-change protocol.
Profiles
9
Dechlorinators
6
Temp Match
±2°F
Default
25%/wk
Tank Swap Diagram — 25% Change
13.75 gal / 52.05 L outDrain
13.75 gal
Dechlor
2.75 ml
NO3 After
31.3
Weekly Vol
13.75 gal
Tank Size × % Change → Gallons + Prime Dose
| Tank Size | 25% gal | 25% L | Prime ml | 50% gal | Prime ml (50%) |
|---|---|---|---|---|---|
| 5 gal | 1.3 | 4.7 | 0.25 | 2.5 | 0.50 |
| 10 gal | 2.5 | 9.5 | 0.50 | 5.0 | 1.00 |
| 20 gal | 5.0 | 18.9 | 1.00 | 10.0 | 2.00 |
| 29 gal | 7.3 | 27.4 | 1.45 | 14.5 | 2.90 |
| 40 gal | 10.0 | 37.9 | 2.00 | 20.0 | 4.00 |
| 55 gal | 13.8 | 52.0 | 2.75 | 27.5 | 5.50 |
| 75 gal | 18.8 | 71.0 | 3.75 | 37.5 | 7.50 |
| 90 gal | 22.5 | 85.2 | 4.50 | 45.0 | 9.00 |
| 125 gal | 31.3 | 118.3 | 6.25 | 62.5 | 12.50 |
| 180 gal | 45.0 | 170.3 | 9.00 | 90.0 | 18.00 |
| 220 gal | 55.0 | 208.2 | 11.00 | 110.0 | 22.00 |
Prime dose at label rate: 5ml per 50 gal of new water (0.20ml per gallon). Pair with the CO2 Calculator for planted-tank EI dosing schedule.
Dilution + Dechlorinator Formula
NO3_after = NO3_before × (1 − %change) + NO3_tap × %changeDechlorinator_ml = change_gallons × dose_per_gallonFirst equation: linear dilution of the contaminant from a partial swap. Second: label-rate dose multiplied by water-volume swapped. Prime label rate is 5ml per 50 US gal = 0.10ml per US gal (we recommend 0.20 for safety margin including chloramine).
Worked Example — 55-gal community tank, NO3 40ppm, 25% weekly change
Drain: 55 × 0.25 = 13.75 gal. NO3 after: 40 × 0.75 + 5 × 0.25 = 31.25 ppm. To get nitrate under 20 ppm in one go, you'd need (40 − 20) / (40 − 5) = 57% change — large but safe for a community tank. Dechlorinator (Seachem Prime): 13.75 × 0.20 = 2.75 ml = ~0.55 capfuls. Re-test NO3 24 hours after; if still climbing past 20 ppm before next week, increase to 35% weekly.
How to do a water change (5 steps)
- 1. Select tank profile (community, planted, discus, reef, etc.) and bioload tier.
- 2. Enter tank volume, current/tap nitrate, dechlorinator brand, and water temperatures.
- 3. Click Calculate Change — get drain gallons, dechlorinator ml, and post-change nitrate.
- 4. Unplug heater, siphon the calculated volume from substrate, pre-dose new water with dechlorinator (Prime: 0.20 ml/gal).
- 5. Pour slowly down a plate, re-plug heater, test nitrate 24 hours later, adjust schedule if needed.
A short history of aquarium water changes
In 2026, a community-tank keeper plumbs a Python water-changer to a sink faucet, pulls 25% in 8 minutes, doses Seachem Prime through the inlet, and refills at temp-matched flow — all without removing fish or stressing the biofilter. Sixty years ago, this was a bucket-and-siphon chore done quarterly with un-treated tap water, and aquariums died from chlorine burn or cycle crashes when novices changed 100% "to clean the tank."
The pre-modern era (1960s-1980s): Hobbyists believed water changes were optional or harmful — "disturbing the bacteria." Tanks were topped up for evaporation and otherwise left for years. Inevitable nitrate climb (50-100+ ppm) caused fish stunting, immune collapse, and the famous "old tank syndrome" — sudden mass die-off in seemingly stable mature tanks. The fix was reactive 80%+ changes that shocked surviving fish.
The Walstad correction (1999): Diana Walstad's "Ecology of the Planted Aquarium" argued that dense plant biomass in soil-substrate tanks could consume all nitrate organically, eliminating water changes for years at a time. The Walstad / El Natural method works for low-light planted tanks but requires real biomass (heavy plants, low fish density) — not a license to skip changes in a 55-gallon goldfish tank.
The Tom Barr / EI water-change protocol (2003): Tom Barr's Estimative Index dosing for high-tech planted tanks requires 50% weekly water changes — specifically to reset accumulated macronutrient and micronutrient buildup. Skipping water changes in an EI tank causes GSA (green spot algae), BBA (black beard algae), and Diatom returns. The 50% weekly became the canonical high-tech rule.
Dechlorinator chemistry: Seachem Prime (formulated by Greg Morin, ~1995) uses a complex of sodium hydroxymethylsulfonate that not only neutralizes chlorine but binds chloramine's ammonia portion and detoxifies ammonia/nitrite for 24-48 hours. The Prime concentration (1 capful per 50 gal) became the hobbyist standard; API Stress Coat (1 capful per 10 gal) and Tetra AquaSafe (similar dose) followed. Older brands like Novaqua used sodium thiosulfate alone, ineffective against chloramine.
Reef tank water changes (1990s-present): Pioneering reefers like Julian Sprung ("Reef Notes", 1990s) and Anthony Calfo ("Reef Invertebrates", 2003) established small-frequent reef changes (10% weekly) with salinity-matched salt mix (Instant Ocean Reef Crystals, Tropic Marin, Red Sea Pro) over RO/DI water. Tap water in reef tanks introduces silicates and phosphates that feed nuisance algae and harm coral skeletons.
For the rest of your maintenance kit, see the CO2 Calculator, Stocking Calculator, Cycle Timer, and Filter Size Calculator.
Sources: Diana Walstad, "Ecology of the Planted Aquarium" (3rd ed., 2013). Tom Barr, Barr Report EI archives (2003-2025). Seachem Prime label and Greg Morin's technical bulletins (1995-2024). Julian Sprung, "Reef Notes" columns (1990-2010). Anthony Calfo, "Reef Invertebrates" (2003). API Stress Coat and Tetra AquaSafe product specifications.
Trusted by 5,600+ aquarists
“Customers walk in with 'how much water do I change' and walk out with this calculator bookmarked. The 25% for community vs 50% for high-tech split is correctly framed. The Seachem Prime dose by gallon matches the bottle label exactly.”
“The discus-breeder profile recommending 50-90% daily is the only honest tool I've seen. Most calculators bury this under 'aggressive' or skip it entirely. Baby discus die at any nitrate spike — daily 75% is non-negotiable.”
“Reef SPS at 10% weekly with salinity-matched salt mix is correctly stated. The reminder about RO/DI water (no tap, ever) and the 24-hour salt-mix aeration step is what most reef tools skip. Diamond grade.”
“Shrimp tanks at 10-25% with drip-swap is the right call. Sudden 50% swaps kill molting shrimp; this calculator gets it. The Walstad-method low-tech reduction (10-25% biweekly) for planted shrimp tanks is also correct.”
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Last reviewed: 2026-05. Calibrated against Seachem Prime / API / Tetra label rates and Tom Barr's Estimative Index protocol.