Water change volume and percentage formulas
Base every calculation on the actual water volume: the water in the tank (and sump) once glass thickness, substrate, rock and decor are taken out. A tank sold as 100 L often holds noticeably less. The calculator can take a total capacity and subtract a share for substrate and decor, but measuring is better.
If water has evaporated since the last top-up, the level is already low. Remove your planned amount and add back that amount plus the evaporated water, so the tank returns to full. In marine and brackish tanks the evaporated part should be fresh RO/DI water, because the salt stays behind.
How much water should you change?
OATA’s care sheets strongly recommend a 25% partial water change at least once a week, with weekly testing for ammonia and nitrite. Veterinary guidance presented at the WSAVA 2018 Congress gives the same 25% figure: once a week in new aquariums and one to two times a month in established ones, with the frequency set by your test results.
Change more often when tests show a problem: ammonia or nitrite above zero, nitrate building up, abnormal pH or alkalinity, contamination, or after medication.
| Real water volume | 25% change | In 10 L buckets | In 5 gal buckets |
|---|---|---|---|
| 40 L (10.6 US gal) | 10 L (2.6 gal) | 1 | 0.5 |
| 100 L (26.4 US gal) | 25 L (6.6 gal) | 2.5 | 1.3 |
| 200 L (52.8 US gal) | 50 L (13.2 gal) | 5 | 2.6 |
| 400 L (105.7 US gal) | 100 L (26.4 gal) | 10 | 5.3 |
The nitrate water change formula
When you swap part of the water, the nitrate left behind is diluted by the new water, which brings its own nitrate with it:
| Tank nitrate | Tap 0 mg/L | Tap 5 mg/L | Tap 10 mg/L |
|---|---|---|---|
| 40 mg/L | 50% | 57% | 67% |
| 60 mg/L | 67% | 73% | 80% |
| 80 mg/L | 75% | 80% | 86% |
One big change or several small ones?
Several smaller changes reach the same nitrate more gently, but they use more water in total. Starting at 40 mg/L with 5 mg/L tap water:
| Plan | Nitrate afterwards | Total water used |
|---|---|---|
| 1 × 57% change | 20.0 mg/L | 57% |
| 2 × 35% changes | 19.8 mg/L | 70% |
| 3 × 25% changes | 19.8 mg/L | 75% |
| 4 × 20% changes | 19.3 mg/L | 80% |
If nitrate has crept up over months, pH and alkalinity have often drifted too. The Merck Veterinary Manual advises fixing that "old tank syndrome" with daily small water changes rather than one large one, to avoid pH shock and a sudden rise in toxic ammonia.
Why your tap water nitrate matters
You can never dilute below the nitrate in your new water. If your tap water carries 25 mg/L, no water change will get the tank to 20 mg/L. Drinking water can legally contain up to 50 mg/L nitrate in the EU and 10 mg/L nitrate-nitrogen (about 44 mg/L nitrate) in the US, so test it, or check your water supplier’s quality report.
Check your test kit’s units too. Some kits report nitrate-nitrogen (NO₃-N). Merck notes that multiplying NO₃-N by 4.4 gives nitrate (NO₃⁻), so 10 mg/L NO₃-N is about 44 mg/L nitrate. The calculator can convert this for you.
Ammonia, phosphate, GH, KH, TDS and salinity
The same mixing formula estimates any dissolved substance straight after a change: C_new = C_tank × (1 − f) + C_new water × f. It assumes the new water mixes in fully and nothing else adds or removes the substance during the change. How well that holds depends on the parameter.
| Parameter | Behaves like a simple contaminant? | Why the real reading can drift |
|---|---|---|
| Nitrate, phosphate | Yes, right after the change | Fish waste and food add more between changes; plants and algae take some up. |
| Ammonia, nitrite | Yes, but should be zero | Fish keep producing ammonia; any reading means the filter is not keeping up. Retest within hours. |
| GH (general hardness) | Partly | Substrate, rocks, remineralisers and plant uptake move it after the change. |
| KH (carbonate hardness) | Partly | Biological filtration uses up carbonates, and CO₂, buffers and substrate change it. pH can shift with KH. |
| TDS | Roughly | Meters estimate from conductivity and lump minerals, salts and fertiliser together. |
| Salinity | Yes, if measured correctly | Rises as water evaporates. SG is not linear, so mix via salinity (ppt) rather than averaging SG readings. |
VIN’s WSAVA proceedings note that biological filtration consumes carbonates, so alkalinity falls over time in tanks without enough water changes. A water change resets it towards the level of your new water, but the tank starts drifting again straight away.
Which replacement water should you use?
| Source | What to do before use |
|---|---|
| Tap water | Treat with a conditioner that removes chlorine and chloramine. Check nitrate and phosphate in your supplier’s report or by testing. |
| RO, RO/DI or distilled | Nearly mineral-free. Remineralise or blend with tap water for freshwater fish; RO/DI is the usual base for marine salt mix. |
| Prepared saltwater | Mix in a separate container to the tank’s salinity and let it circulate until fully dissolved (Instant Ocean: at least overnight). |
| Well water | Usually chlorine-free but can carry nitrate, metals or very hard water. Test first. |
| Rainwater | Soft and acidic, and roofs and gutters add contaminants. Test and buffer before use. |
Matching the new water temperature
Match the new water to the tank as closely as you can and check both with a thermometer. Straight after refilling, the tank sits near T_tank × (1 − f) + T_new × f, so a 25% change with water 3 °C cooler drops a 25 °C tank to about 24.3 °C before the heater catches up.
There is no single temperature difference that is safe for every species. Small tanks and sensitive species (bettas, fry, shrimp) react fastest, so keep the difference small and add the water slowly. Some keepers use slightly cooler water on purpose to trigger spawning; only do that deliberately.
Marine, reef and pond water changes
Instant Ocean’s directions for its sea salt are to change 20% of the aquarium water every two weeks, and never to mix salt in an aquarium that contains animals. Prepare the new saltwater ahead, match its salinity and temperature to the tank, and top up evaporation with fresh RO/DI water only.
For ponds, OATA’s care sheet recommends testing the water weekly, keeping nitrate no more than 20 mg/L above your normal tap water level, and treating all tap water that enters the pond with a conditioner, including hose top-ups. Rinse filter media in pond water removed during a partial change, not under the tap.
Planning a water change schedule
Pick a starting routine for your tank type, then let test results adjust it. Heavily stocked tanks, goldfish and big cichlids usually need more water or more frequent changes; lightly stocked planted tanks may need less. The calculator lists your next change dates and how much water you will use each month and year.
| Tank type | Starting routine | Basis |
|---|---|---|
| Freshwater, community, goldfish, betta | 25% weekly | OATA care sheets; WSAVA 2018 (1–2× a month once established) |
| Marine and reef | 20% every two weeks | Instant Ocean directions |
| Ponds | Test weekly; change to keep nitrate ≤ 20 mg/L above tap | OATA pond care sheet |
| Shrimp | Small, carefully matched changes (about 10% weekly) | General hobby guidance |
| Hospital / quarantine | Whenever ammonia or nitrite reads above zero | Test daily; follow medication labels |
What nitrate level should you aim for?
| Parameter | Freshwater | Saltwater |
|---|---|---|
| Ammonia (total ammonia nitrogen) | 0 mg/L | 0 mg/L |
| Nitrite | 0 mg/L | 0 mg/L |
| Nitrate | below 20 mg/L | below 70 mg/L |
| pH | 6.5–9.0 | 7.8–8.3 |
Freshwater fish tolerate nitrate much better than marine fish and invertebrates. The WSAVA 2018 proceedings note that most freshwater fish cope with 200 mg/L for short periods, but recommend water changes once nitrate passes 20 mg/L, partly because high nitrate feeds algae.
How to do a big water change safely
- Treat every litre of new water with a dechlorinator. Chloramine in tap water releases ammonia when it is neutralized, so use a conditioner that binds ammonia as well.
- Match the temperature of the new water to the tank as closely as you can.
- Keep pH similar. Large swings stress fish. Where nitrate is very high and pH has fallen, change water in daily steps.
- Rinse filter media in removed tank water, never under the tap, so the beneficial bacteria survive (OATA).
- Retest afterwards and repeat if the numbers are still off.
Ammonia emergency
