Aquarium water change schedule
How much water to change, and how often, depends on how fast nitrate builds up in your tank. Here is a schedule for common tank types, worked out from the nitrate math.
Schedule by tank type
Each row starts from a typical weekly nitrate rise for that kind of tank and a nitrate level to stay under, then works out the share of water to change. With nitrate-free tap water, a weekly change of p holds the peak at rise ÷ p, so p = rise ÷ peak; two changes a week each need half that.
| Tank | Nitrate rise / week | Keep under | Once a week | Or twice a week | Settles between |
|---|---|---|---|---|---|
| Lightly stocked community A few small fish per 10 gallons, moderate feeding. | 5 ppm | 20 ppm | 25% | 2 × 15% | 15.0–20.0 ppm |
| Moderately stocked community A typical community tank. | 10 ppm | 30 ppm | 35% | 2 × 20% | 18.6–28.6 ppm |
| Heavily stocked, goldfish or cichlids Large or messy fish, heavy feeding. | 20 ppm | 40 ppm | 50% | 2 × 25% | 20.0–40.0 ppm |
| Heavily planted, light stocking Fast-growing plants take up part of the nitrogen. | 3 ppm | 20 ppm | 15% | 2 × 10% | 17.0–20.0 ppm |
| Shrimp tank Shrimp prefer low nitrate and small, gentle changes. | 3 ppm | 10 ppm | 30% | 2 × 15% | 7.0–10.0 ppm |
| Betta or nano tank (under 10 gal) Little water, so waste concentrates quickly. | 8 ppm | 20 ppm | 40% | 2 × 20% | 12.0–20.0 ppm |
| Fry grow-out Heavy feeding for fast growth. | 15 ppm | 20 ppm | 75% | 2 × 40% | 5.0–20.0 ppm |
Percentages are rounded up to the next 5%. The weekly rises are typical starting values, not measurements: tanks with the same fish vary several-fold with feeding, filtration and plants. Measure your own by testing nitrate just after a change and again a week later, then put that number into the calculator above.
- Over about 50% a week: split it into two changes (the fry row needs 75% once a week, or 40% twice) — smaller changes are gentler on fish and on water chemistry.
- Shrimp: keep changes small and frequent, match temperature and hardness carefully, and drip new water in slowly.
- Planted tanks: plants use nitrate, and heavily fertilized tanks may deliberately keep 10–20 ppm. Water changes there also reset fertilizer build-up.
Where nitrate peaks: rise × change size
The level just before each weekly change, with nitrate-free tap water:
| Rise / week | 10% | 20% | 25% | 30% | 40% | 50% |
|---|---|---|---|---|---|---|
| 5 ppm | 50 ppm | 25 ppm | 20 ppm | 17 ppm | 13 ppm | 10 ppm |
| 10 ppm | 100 ppm | 50 ppm | 40 ppm | 33 ppm | 25 ppm | 20 ppm |
| 15 ppm | 150 ppm | 75 ppm | 60 ppm | 50 ppm | 38 ppm | 30 ppm |
| 20 ppm | 200 ppm | 100 ppm | 80 ppm | 67 ppm | 50 ppm | 40 ppm |
Most freshwater fish do well under about 40 ppm; fry, shrimp and sensitive species are better under 20 ppm. Tap water sets a floor: with 10 ppm in the tap, a 30% weekly change on a tank rising 10 ppm a week peaks at 43.3 ppm instead of 33.3 ppm.
How much water is that?
Gallons to remove for standard tanks, using the real water with 2 inches of gravel, and the number of 5-gallon buckets:
| Tank | Real water | 25% | 35% | 50% |
|---|---|---|---|---|
| 5 gallon | 3.6 gal (14 L) | 0.9 gal (1 bucket) | 1.3 gal (1 bucket) | 1.8 gal (1 bucket) |
| 10 gallon | 7.4 gal (28 L) | 1.8 gal (1 bucket) | 2.6 gal (1 bucket) | 3.7 gal (1 bucket) |
| 20 gallon long | 13.1 gal (50 L) | 3.3 gal (1 bucket) | 4.6 gal (1 bucket) | 6.6 gal (2 buckets) |
| 29 gallon | 22.1 gal (84 L) | 5.5 gal (2 buckets) | 7.7 gal (2 buckets) | 11.0 gal (3 buckets) |
| 40 gallon breeder | 34.3 gal (130 L) | 8.6 gal (2 buckets) | 12.0 gal (3 buckets) | 17.1 gal (4 buckets) |
| 55 gallon | 45.6 gal (173 L) | 11.4 gal (3 buckets) | 16.0 gal (4 buckets) | 22.8 gal (5 buckets) |
| 75 gallon | 62.2 gal (235 L) | 15.5 gal (4 buckets) | 21.8 gal (5 buckets) | 31.1 gal (7 buckets) |
| 125 gallon | 93.8 gal (355 L) | 23.4 gal (5 buckets) | 32.8 gal (7 buckets) | 46.9 gal (10 buckets) |
Above about 40 gallons a hose-based water changer that drains to a sink and refills from the tap saves a lot of carrying — add dechlorinator for the whole tank volume when you refill straight from the tap. Scale your conditioner with the dosing calculator.
How often, in short
- Weekly is the default. A fixed day makes it a habit.
- Every two weeks works only for lightly stocked or heavily planted tanks whose nitrate stays low — test to confirm.
- Twice a week for fry, heavily stocked tanks and shrimp tanks on small changes.
- After a long gap, bring nitrate down over several smaller changes rather than one huge one.
The maths and the step-by-step method are in the water change guide; the water change calculator shows how many changes it takes to reach a target.
FAQ
How often should I change aquarium water?
Weekly suits most tanks. A moderately stocked community tank usually needs about a third of its water changed every week to keep nitrate near or under 30 ppm; lightly stocked and planted tanks can need less.
Is it better to do one big or two small water changes a week?
For the same total, two smaller changes keep nitrate lower and steadier than one big change, and are gentler on sensitive fish and shrimp. One weekly change is easier and works well for most tanks.
How do I know how fast nitrate rises in my tank?
Test nitrate right after a water change and again a week later, before the next change. The difference is your weekly rise. Use it in the water change calculator to see where your routine settles.
What if my tap water has nitrate?
Tap nitrate sets a floor: water changes can never bring the tank below it. Aim for a peak above your tap value and consider plants, lighter feeding or filtered water if your tap is high.