Last Updated: September 29, 2026
There is no tidy chart that turns tank gallons into an ATO size. Answering what size auto top off system do you need means sizing three things separately: a reservoir from measured evaporation times the days you want between refills, a pump that clears your real vertical lift, and a controller that stays within any stated tank-size ceiling.
As an Amazon Associate we earn from qualifying purchases.
By David Nguyen
Quick answer: Our top pick in 2026 is the Daily evaporation — our #1 rated choice. See the full ranked comparison, alternatives and buying advice below.
How to Size an Auto Top Off System
Sizing an ATO is not a tank-gallon lookup; it is three separate calculations from measured inputs. The table and criteria below cover them: measure daily evaporation first, calculate reservoir volume from your desired days, size the pump separately by head, and check any explicit controller size limit before buying.
| Measured Input | Example | Calculation / Test | Decision |
|---|---|---|---|
| Daily evaporation | 0.6 gal/day | x 7 days = 4.2 gal | About a 5-gal reservoir class |
| Daily evaporation | 1.2 gal/day | x 7 days = 8.4 gal | About a 10-gal reservoir class |
| Vertical lift | 9 ft | Compare to pump max head | Drop pumps below 9 ft |
| Tank / controller scope | 75 gal | Compare to explicit ceiling | Exclude Tunze Nano 66-gal |
Measure Daily Evaporation First
The primary number is measured daily evaporation, not display gallons. Bulk Reef Supply identifies surface area, water movement, temperature and humidity as the main variables, so measuring the actual water lost per day over normal operation is what sets the whole sizing exercise on a real footing.
So record the daily drop over representative operation before anything else, since it feeds both the reservoir and the pump duty. Because evaporation, not tank volume, is what the system actually replaces, this single measurement anchors a sizing method that a gallons-based lookup table simply cannot.
Calculate Reservoir Volume From Days of Autonomy
The reservoir is a volume, found by multiplying evaporation by the days you want between refills. The formula is gallons per day times days, which gives gallons, so 0.6 gallons a day over seven days is about 4.2 gallons, while 1.2 gallons a day over seven days is about 8.4 gallons.
So decide your desired autonomy, then multiply, keeping the result as a volume rather than a flow. The unit check matters: gallons per day times days equals gallons, which is why 8.4 gallons of reservoir demand must never be misread as 8.4 gallons per hour of pump flow by mistake.
Size the Pump Separately by Vertical Head
The pump is sized by lift, entirely separately from the reservoir. A reservoir can hold plenty of water while the pump still cannot reach the sump, so the vertical rise is matched against each model’s maximum head rather than against the reservoir’s capacity or the tank’s gallons.
So treat pump sizing as its own step keyed to head, not volume. At a 9-foot lift, an 8-foot ReefATO+ or an 8.2-foot Duetto2 falls outside range while the full Tunze Boost’s 20.34 feet stays inside, and even then head eligibility is not a promise of delivered flow at that full height.
Check Any Explicit Controller Size Limit
Finally, check whether the controller carries a stated tank-volume ceiling. The Tunze Nano is scoped to about 66 gallons, so a 55-gallon system sits inside its published range while a 75-gallon one sits outside, and many other ATOs publish no tank-gallon ceiling of any kind at all.
So apply an explicit ceiling where one exists, and ignore gallons where it does not. This controller eligibility is separate from reservoir gallons and pump head, which is why the three checks stay independent: a size ceiling constrains the controller, not the reservoir you pair with it.
Adjust for Your Situation
Once the three sizes are set from measurement, a few common situations shift the details rather than the method. The four below cover them: a typical mid-size aquarium, wanting a full week between refills, a reservoir sitting far below the sump, and wanting extra capacity for a vacation.
If You Have a 50 to 100 Gallon Aquarium
For a common mid-size tank, treat published rules of thumb as context, not a size. Bulk Reef Supply notes that many 50-to-100-gallon aquariums may use roughly a 5-gallon jug a week, but it stresses that evaporation varies, so that figure is a starting reference rather than a prescription.
So measure your own evaporation before buying the reservoir, even on a typical tank. The 5-gallon-a-week example is a helpful sanity check, not a lookup value, which is why your measured daily loss over seven days, rather than the tank’s gallons, is what should actually size the container.
If You Want a Full Week Between Refills
If seven days of autonomy is the goal, multiply measured evaporation by seven, not by tank gallons. At 0.6 gallons a day that is about 4.2 gallons, and at 1.2 gallons a day it is about 8.4 gallons, so the same desired week produces very different reservoirs on two different tanks.
So encode the week you want as the days figure in the formula, and let evaporation do the rest. Because the reservoir scales with real water loss rather than the tank’s size, two seven-day setups can need quite different containers, which is exactly why the method beats a fixed gallons rule.
If the Reservoir Sits Far Below the Sump
If the reservoir sits well below the sump, measure the vertical rise and the tubing run carefully. Red Sea explicitly notes that locating its reservoir far from the sump reduces pump flow, so a long, tall run can leave a nominally adequate pump struggling to deliver at that height.
So size the pump for the real lift and distance, not just the reservoir volume. A big container does not help at all if the pump cannot push water up to the sump, which is why a far or low reservoir makes the head check, and an honest tubing measurement, more important rather than less.
If You Want a Larger Reservoir for Vacation
If you want more days for a trip, add them as explicit autonomy days rather than a vague buffer. Multiplying measured evaporation by a larger number of days sizes the extra capacity honestly, instead of hiding it behind an invented percentage margin bolted on top of the base figure.
So state the extra days in the formula and keep the fail-safes intact, since more stored water raises the stakes. A bigger reservoir holds more water that a malfunction could move, which is why overfill and siphon protection matter more, not less, as autonomy grows for a vacation.
Frequently Asked Questions
These answers keep sizing tied to measured evaporation and lift, since tank gallons alone cannot set an ATO size. Each opens with the direct answer and the reason behind it, and keeps the result a volume in gallons, never a flow in gallons per hour, and never an invented reserve percentage.
What Size ATO Reservoir Does a 75-Gallon Tank Need?
It cannot be answered from 75 gallons alone, because tank volume does not set evaporation. If your measured evaporation is 1 gallon a day and you want seven days between refills, the calculation is simply 7 gallons of reservoir, regardless of what the display tank itself actually happens to hold.
So run your own measured number through the formula rather than reading off the tank size. Two 75-gallon tanks with different surface area, temperature or cover can need quite different reservoirs, which is exactly why the gallons on the glass are the wrong input for this question.
Is a 5-Gallon ATO Reservoir Enough?
It depends entirely on your own measured evaporation rate, not on the tank size. At 0.6 gallons a day, 5 gallons covers about 8.3 days in theory, while at 1.2 gallons a day it covers only about 4.2 days, so the same container gives very different autonomy on two otherwise different tanks.
So divide 5 gallons by your daily loss to see the real days, and expect the usable figure to be lower. Unusable residual volume in the container and unexpected evaporation swings both shorten it, which is why the honest answer comes from your measured rate rather than the tank’s size.
Should the Pump GPH Match Aquarium Gallons?
No, because pump choice depends mainly on head and reliable delivery, not on the tank’s volume. Matching gallons-per-hour to aquarium gallons is a coincidence of numbers, since the pump only needs to clear your vertical lift and replace slow evaporation, not move water on demand.
So size the pump by the rise it must overcome and the water it must deliver. A pump that reaches your outlet and keeps up with real evaporation is correctly sized whatever its maximum flow, which is why the gallons-to-gallons match is simply not a real sizing method for the pump at all here.
Is a Bigger Reservoir Always Better?
No, there is no universal rule that bigger is better, because size should follow measured demand and desired autonomy. A container far larger than your evaporation and refill interval require adds cost and bulk without a real benefit, and it does not improve how the pump or controller performs.
So choose the reservoir for your measured demand and the days you want, while keeping overfill protection and safe plumbing. A larger reservoir also stores more water that a fault could release, which is why growing capacity sensibly, rather than maximally, is the better sizing habit.
Final Thoughts
Answering what size auto top off system do you need is three separate sizes, not a tank-gallon chart: a reservoir from evaporation times your desired days, a pump for your real lift, and a controller within any stated size ceiling. Keep the result a volume, not a flow, and add days rather than an invented percentage.
Ready to decide? Our #1 pick for 2026 is the Daily evaporation.
Live price & availability on Amazon.
Related Reviews & Guides
Fish Tank Stand and Aquarium Cabinet Buyer’s Guide
4 Best Aquarium Auto Feeders for Vacation
