What daily water change equals 20% weekly?
One 3.14% change a day matches a single 20% change over a week. That is 6.27 L a day on a 200 L system. Dividing 20 by seven spends the same water, but achieves only 18.37% dilution: each later drain takes out some of the fresh water you already added.
The calculator starts with the routine you want to replace. Choose weekly or a 30-day month, enter its percentage, then select how your equipment exchanges water. If your system is already running, choose “I know my daily %” to find its effective weekly and monthly replacement. Percentages use the actual connected water volume, including the sump. A nominal tank size can overstate the display's water while leaving the sump out entirely; use true system volume for the pump setting.
“Equivalent” here means the same fraction of an existing, well-mixed dissolved substance is removed over the selected period, assuming none is produced or consumed during that calculation. It does not mean the fish experience identical conditions. The separate nitrate comparison accounts for ongoing production and shows the difference.
Is 1% daily the same as 7% weekly or 30% monthly?
No. One percent daily removes 6.79% of the original tracer after a week, and 26.03% after 30 days. It still consumes 7% and 30% of the tank's volume in new water. Those are two different measurements: the amount you mix and pump, and the fraction of the original water's dissolved contents you remove.
After a 1% change, 99% remains. After the next, 99% of that remains. Multiply the remaining fractions, then subtract from one. For a daily fraction d and n days:
effective replacement = 1 − (1 − d)n
The inverse is d = 1 − (1 − target)1/n. Enter percentages as fractions: 20% is 0.20. These equations describe drain-and-refill changes that mix completely between cycles.
This is a small efficiency penalty for a modest routine, not a reason to dismiss automation. The 20%-weekly example uses 9.8% more water when converted to matching daily changes. A schedule you can sustain may be worth that extra water. A missed manual change is a much bigger difference than the mixing penalty.
Daily, weekly, and monthly water change equivalence chart
Each row assumes one drain followed by a refill every day. The last column is water consumed; the middle columns are effective replacement. For several cycles per day or simultaneous pumps, use the tool above.
| Daily change | Weekly equivalent | 30-day equivalent | Water used / 30 days |
|---|---|---|---|
| 0.25% | 1.74% | 7.23% | 7.5% |
| 0.5% | 3.45% | 13.96% | 15% |
| 1% | 6.79% | 26.03% | 30% |
| 1.5% | 10.04% | 36.45% | 45% |
| 2% | 13.19% | 45.45% | 60% |
| 3% | 19.2% | 59.9% | 90% |
| 4% | 24.86% | 70.61% | 120% |
| 5% | 30.17% | 78.54% | 150% |
One drain-and-refill per day; full mixing between changes. Equivalents describe removal of an existing dissolved tracer, with no new input. Month = 30 days. Computed by AquariumToolbox's water-change engine.
To match 20% weekly: 3.14% daily. Daily liters = true system liters multiplied by daily percentage / 100. For 200 L: 6.27 L/day. Simultaneous pumps require 3.19% daily instead.
Does pumping water in and out at the same time waste it?
Some new water leaves with the old when the tank mixes during pumping. The amount is predictable. For equal simultaneous inflow and outflow in a well-mixed system, effective replacement is 1 − e−Q/V, where Q is total water pumped and V is system water. This is the continuous-exchange model in Randy Holmes-Farley's water-change analysis.
Matching 20% weekly takes 3.19% of the tank per day with simultaneous pumps, compared with 3.14% as one daily drain-and-refill. On the 200 L example, that is 6.38 versus 6.27 L/day. Splitting sequential changes into more cycles nudges their water requirement toward the simultaneous value, because each cycle has another chance to remove recently added water.
The model assumes the intake samples mixed tank water. Put the new-water outlet where it can mix into the system, away from the waste pickup. If the waste line catches a stream of new saltwater directly, the real efficiency can be worse than this calculation. An overflow or drain-through system with very little mixing can behave differently too; do not treat it as a calibrated dosing-pump exchange.
Are daily changes better for nitrate than weekly changes?
They usually narrow the swing; the average also depends on the amount changed. At a constant rise of 1 ppm/day and zero nitrate in the new water, the 200 L example eventually reaches these repeating ranges:
| Equivalent schedule | Nitrate low–high | Average | New water / 30 days |
|---|---|---|---|
| One 20% change per week | 28–35 ppm | 31.5 ppm | 171.4 L |
| One equivalent change per day | 30.87–31.87 ppm | 31.37 ppm | 188.2 L |
| Simultaneous pumps, 24 cycles/day | 31.35–31.39 ppm | 31.37 ppm | 191.3 L |
Computed from constant net production, complete mixing, and evenly spaced short exchanges. Simultaneous-pump cycles are treated as brief mixed exchanges; truly continuous pumping approaches a flat level. These are eventual ranges, not a forecast from today's reading.
The daily example still peaks at 31.9 ppm, despite a swing of only 1 ppm. A stable high reading is possible. If the goal is a particular nitrate ceiling, use the water-change schedule calculator to size that first, then convert the routine here.
The model reuses the site's steady-state mixing engine. Food, plants, feeding changes, denitrification, and test uncertainty affect the real net rise. New-water nitrate adds a floor to both schedules. If production is zero, a nonzero exchange eventually approaches the source concentration; with no exchange, there is no unique equilibrium for the tool to predict. Test at consistent times when comparing readings.
How many mL should I set on my DOS or other dosing pump?
Each head moves the full per-cycle amount. One removes it; the other replaces it. Do not divide that figure in half because there are two heads. In the simultaneous example with 24 cycles a day, set each exchange to 265.65 mL per head. If your controller asks for a daily total, use the daily volume instead and let its scheduling routine distribute it.
Calibrate both heads with the installed tubing and lift, prime out the air, and measure the actual amount collected. Neptune's DOS manual calls for calibrating each pump and recalibrating after changes to the fluid or tubing. The optional runtime estimate uses your measured flow: sequential drain-and-refill needs both heads' times added; simultaneous operation takes the longer head's time. The calculator flags schedules that cannot fit into a day, but it cannot know a particular pump's permitted duty cycle.
Keep freshwater top-off separate from saltwater exchange. During a sequential drain, prevent the ATO from filling the temporary gap with fresh water. Use the controller's supported interlocks and level protection, and check that the replacement reservoir cannot empty or the waste reservoir overflow. Check salinity and the actual volumes periodically: matching settings do not prove two pumps still deliver equal amounts.
How much extra salt does automatic water changing use?
Salt is charged against water mixed, not effective replacement. For the 200 L example, the matching daily routine uses 188.2 L per 30 days versus 171.4 L averaged over weekly changes. At an illustrative 40 g of mix per liter and 5 currency units per kilogram, daily changes use 7.53 kg and cost 37.65. The extra salt costs 3.36 per 30 days.
Use your own label's yield and bag price. The salt-mix calculator handles brands and salinity; this budget assumes the finished replacement water is already correct for the tank. RO production, electricity, equipment, and labor are outside the salt-only total. The larger the routine you are matching, the larger the mixing penalty: matching 50% weekly with one daily change uses 32% extra water.
Why has nitrate stayed high since I switched to AWC?
First check whether the setting preserved the old water volume or its dilution effect, then measure both pump outputs. Confirm the true system volume, test the new water, and compare the daily setting with the tool's long-term range. If feeding or plant growth changed, your old nitrate-production estimate may no longer fit. Frequent small exchanges maintain a routine; they do not instantly erase a large existing buildup.
For a reading that needs correcting now, use the nitrate correction tool, or the reef water-change comparison when several parameters matter. Prepare freshwater with the appropriate conditioner dose, and check replacement temperature and chemistry. Automatic water exchange also leaves detritus in the tank: siphoning and filter maintenance remain separate jobs.