How Often to Change Aquarium Water Calculator
Not sure how often to change aquarium water? This free aquarium water change calculator builds a schedule for your tank from three things: tank size, bioload (how many fish and how much they eat), and whether the tank is planted. It outputs the recommended change percentage, the frequency, and exactly how many gallons to swap per change and per month.
Enter your numbers
Light = a few small fish, lightly fed. Heavy = fully stocked, messy eaters, or modest filtration.
Live plants absorb nitrate and waste, so planted tanks can go lighter on water changes.
Your results
How to use this calculator
Enter your tank's volume in gallons. If you don't know it, measure your tank and use our aquarium tank volume calculator — this page is about when and how much water to change, not how big the tank is.
Pick the bioload that best matches your stocking. A handful of tetras that get a pinch of food is light; a fully stocked cichlid tank with big eaters is heavy. Then tell us whether the tank has live plants. Your recommended schedule updates instantly.
For a heavily stocked, unplanted tank, consider splitting the recommended weekly change into two smaller changes (for example, 20% twice a week instead of 35% once) — smaller, more frequent changes keep water chemistry more stable.
How it works
The calculator maps bioload and plants to a standard hobbyist schedule: light, non-planted tanks get 20% weekly; light planted tanks get 20% every two weeks; medium non-planted tanks get 25% weekly; medium planted tanks get 20% weekly; heavy planted tanks get 30% weekly; and heavy non-planted tanks get 35% weekly. These ranges follow the widely used guideline of 20–30% weekly changes for typical tanks, going lighter for planted tanks and heavier for high-bioload tanks.
Gallons per change = tank size × change percentage. Monthly volume assumes 52 weeks per year divided by 12 months (about 4.33 changes per month for a weekly schedule). For example, a 20-gallon tank with medium bioload gets 25% weekly: 20 × 0.25 = 5 gallons per change, and 5 × 4.33 ≈ 21.7 gallons per month.
These are estimates, not rules carved in stone — real tanks vary with feeding habits, filtration, temperature, and species. Use a water test kit for nitrate: if nitrate stays under 40 ppm with this schedule, you're on track; if it climbs, change more often or feed less.
Frequently asked questions
How often should I change aquarium water?
Most tanks do best with a 20–30% partial water change once a week. Lightly stocked or planted tanks can often stretch to every two weeks, while heavily stocked tanks may need 30–40% weekly or two smaller changes per week.
How much water should I change at once?
Change 20–30% of the tank's volume per water change. Larger changes (over 50%) should be avoided except in emergencies, because sudden shifts in temperature, pH, and chemistry can stress or harm your fish.
Do planted aquariums need fewer water changes?
Live plants absorb nitrate and some waste, so heavily planted tanks can usually get by with a smaller or less frequent schedule, such as 20% every two weeks. Plants still consume minerals and organic waste accumulates, so planted tanks still need regular changes.
What is aquarium bioload?
Bioload is the total waste load in your tank, driven by how many fish you keep, their size, how much you feed, and how efficient your filtration is. More fish and more food mean more ammonia and nitrate, which means more frequent or larger water changes.
Should I vacuum the gravel during water changes?
Yes, siphoning the substrate during a water change is the easiest way to remove uneaten food and fish waste, which is the main source of nitrate buildup. In heavily planted tanks, vacuum lightly or only the open areas to avoid disturbing plant roots.