Auto Top-Off Reservoir Size Is a Refill Interval

The 3.6 gallons a day that should have been 7

Melev's Reef sized a 45 gallon top-off reservoir for a 400 gallon reef, aiming at roughly a week of supply against an anticipated 7 gallons of evaporation a day. Measured over the first year, that tank actually lost 3.6 gallons daily. Melev's Reef sized to a refill interval rather than to a percentage of volume, and the estimate was off by roughly a factor of two.

Built from published specs, hobbyist builds and forum-documented results, with sources named in the text. Links may earn a commission, which has no bearing on what gets recommended.

The error was in the safe direction. It still tells you something. A reservoir sized for seven days held closer to fourteen, because the anticipated rate overshot the measured one by roughly 100%.

That guess is what this article is about. If you are running a reef or a heavily lit planted tank and picking a container for top-off water, you are choosing between a footprint you have to live with and a refill schedule you have to keep. Melev's own numbers put a price on getting it wrong the other way: two 20 gallon containers meant a 15 gallon refill every three days for six years, around 2,190 trips.

By the end you will have your own evaporation figure in gallons per day, a target refill interval in days, and a check against the failure ceiling the sellers publish. The 45 gallon acrylic vessel that solved it measures 22" x 15.5" x 32". That is the trade.

The short version

• The pick: Inland Seas Simplicity 10, for you want the longest possible stretch between refills on an evaporation-heavy reef tank and have sump-cabinet floor space to give it.
• Also worth it: Bashsea 5, for you need a reservoir that slides into a narrow gap beside a sump rather than sitting in open floor space.
• How this was decided: published specifications and buyer reports across 6 products, compared below.

Two numbers, in this order

A 75 gallon mixed reef losing 0.4 gallons a day, sized for a week between refills, needs 2.8 gallons. Applying the sellers' 15-20% range, the ceiling on that system is 11.25 to 15 gallons, so the interval number binds — though no source supplies evaporation rates by tank size, so the 0.4 gallon figure is an illustration rather than a measured one.

Two calculations, in this order:

• Target volume = measured daily evaporation × desired refill interval. Take the evaporation figure from your own tank rather than from a chart band. Reef Casa makes the same point: refill frequency is a function of the evaporation rate, so nothing converts a reservoir size into days without that measurement.
• Ceiling = 15–20% of total system volume. Both Bulk Reef Supply and Reef Casa land on this range. Reef Casa's reasoning is failure, not convenience: a sensor or pump failure dumping the entire reservoir should not cause a fatal salinity swing or an overflow.
• If target exceeds ceiling, the ceiling wins. Shorten the interval rather than buying the larger vessel. Seven days is a preference. Livestock is not.
• On smaller tanks, the interval will often bind first. A 40 gallon tank has a 6–8 gallon ceiling under the sellers' range.
• On very large systems the ceiling is far looser. A 400 gallon system evaporating 3.6 gallons a day wants 25 gallons for a week, and 60 to 80 gallons would sit inside the sellers' range — but commercial reservoirs top out at 15 gallons, sized against neither number.

Run both. Take the smaller. The order matters because the ceiling is a veto, not an input.

Run the salinity drop yourself

No source available here gives the salinity math, so the following is arithmetic rather than anything a vendor publishes. Take a 90 gallon display with a 20 gallon sump, running roughly 100 gallons of actual water. Dump a full 10 gallon reservoir into it and salinity falls about 9%: 1.026 specific gravity drops to somewhere near 1.0236. Run the same failure with a 15 gallon reservoir and you are near 1.0225.

The arithmetic is simple enough to do on a phone. New salinity ≈ old salinity × (system volume ÷ (system volume + reservoir volume)), working in ppt or in the decimal part of specific gravity. On 100 gallons of system water, 5 gallons costs you roughly 0.0012 SG, 10 gallons about 0.0024, 15 gallons about 0.0036. On a 40 gallon system — a nano with a 10 gallon sump — a 5 gallon dump takes 1.026 to about 1.0231, and a 10 gallon dump takes it to 1.0208.

Whether the 15-20% figure is a measured threshold or a round number is not something either seller shows: neither Reef Casa nor Bulk Reef Supply publishes the calculation, cites a test, or says where the range came from. What Reef Casa does state is that an oversized reservoir risks a sudden salinity drop that can kill fish and corals and crash the tank. Bulk Reef Supply frames the same risk as excessive fresh water being added accidentally during an ATO failure or overfill.

Overflow is the other failure the sellers name: Reef Casa describes an oversized reservoir overflowing the tank as a mess and a potential fire hazard, and treats it alongside the salinity risk rather than ranking the two. Neither source relates the ceiling to a sump's empty headroom, or says which of the two ceilings binds first.

Why Bulk Reef Supply's chart disagrees with Bulk Reef Supply

Bulk Reef Supply publishes two sizing rules that do not agree with each other. Its written ceiling, in the ATO guidance attributed to Robert Farnsworth and last updated 23 October 2024, is that a reservoir should hold no more than 15-20% of the aquarium's total water volume — though no page defines whether that means display volume, display plus sump, or water volume net of rock and sand. Its chart, on the same page, pairs a 5 gallon reservoir with tanks up to 75 gallons. That is 6.7%.

The rest of the chart holds the same line. Ten gallons is matched to tanks up to 120 gallons, roughly 8.3%; fifteen gallons is matched to 200+ gallons, which lands under 8% and falls further the larger the system gets. So the seller's own recommendations sit at roughly half the percentage ceiling it publishes — a discrepancy no page reconciles.

The percentage ceiling is stated as a failure limit: how much fresh water can enter before a sensor or pump failure drops salinity to a fatal level. What the chart is sized against is not stated anywhere — either it is conservative for a reason the seller does not give, or the percentage rule is not the one it actually applies.

When they conflict, the ceiling is the number with a stated consequence attached: Bulk Reef Supply says a too-small reservoir means frequent manual refills and risks burning out a pump run dry, while both sellers tie oversizing to salinity crash and overflow.

Product	Material	Item Weight	Color	Material Type	Suits
Inland Seas Simplicity 10	Plastic, Acrylic	14.6 pounds	black	Plastic, Acrylic	Best long-interval capacity
Bashsea 5	Acrylic	—	White	Acrylic	Best narrow footprint
Marinecolor® Acryli Made	Acrylic	3.1 pounds	Clear	Acrylic	Best multi-compartment
Useek	Acrylonitrile Butadiene Styrene	0.4 kg	Green-1	Acrylonitrile Butadiene Styrene	Best discreet sensor
EPTTECH	Acrylic	—	Black	Acrylic	Best no-probe install
Useek	Acrylonitrile Butadiene Styrene (ABS)	0.4 kg	Blue	Acrylonitrile Butadiene Styrene (ABS)	Best colour match

Spec columns quote the listings verbatim. “Suits” is editorial.

Sort it in ten seconds

• Inland Seas Simplicity 10, not for you if this matters: 20.5 x 16 x 22.5 in footprint and 22.5 in height won't clear many stand openings.
• Bashsea 5, not for you if this matters: white acrylic means no external view of the water level.
• Marinecolor® Acryli Made, not for you if this matters: 4.5 L total is very small for top-off duty — daily or near-daily refills on most tanks.

The floor below which the pump runs dry

There is also a floor, though no source sets it at a particular number of days. Bulk Reef Supply puts the consequence plainly: too small a reservoir means frequent manual refills, and running the pump dry in an empty reservoir can burn it out. If you take three days as your own minimum, then at 0.5 gallons a day that is 1.5 gallons; at 3.6 gallons a day, it is nearly 11.

Two things remove the floor entirely.

One is a plumbed feed. Melev's Reef runs a reservoir fed directly by a 150gpd boosted RO/DI system, which removes jug hauling from the equation entirely and changes what size is adequate. Whether a continuously refilled small reservoir is in fact safer than a large manual one is not something any vendor page addresses.

Neither is available to most people with a stand and a jug. Run-dry protection is a listed feature on Bulk Reef Supply's ATO systems, and it matters more the smaller the reservoir, because a small reservoir empties sooner.

What the market actually offers, and at what price

Commercial reservoirs cluster at 5 and 10 gallons, with 15 gallons at the top of the range. Innovative Marine's Auqa Gadget 15 Gallon Hydrofill lists at $264.00 against $172.00 for the 5 gallon in the same line — roughly $92 for ten additional gallons. Simplicity's Easy-Fill runs $149.95 at 5 gallons and $219.95 at 10, a $70 premium for doubling capacity. Trigger Systems' Ruby ATO 10 Gallon lists at $214.79, Pro Clear Aquatic Systems' 10 Gallon at $162.54, and Red Sea's ReefCan holds 4.8 gallons (18 litres) at $75.00, the cheapest small purpose-built option listed. The cheapest reservoir Bulk Reef Supply lists at all is Flipper's Gravity-Fed at $59.99 — though gravity-fed and pumped reservoirs are different products with different sizing implications, and the price list does not distinguish them.

What none of these listings give is dimensions or stand clearance, which is often the real constraint on size. The only dimensional figure available anywhere here is a custom build: Melev's Reef's 45 gallon acrylic reservoir occupies 22" x 15.5" x 32" in 3/8" cast acrylic. Bulk Reef Supply's storefront organises ATO systems by recommended tank size, in bands from '5 Gallons or Less' to 'Over 600 Gallons'. Reef Casa instead sizes reservoirs to match specific tanks in its lineup, and its slim and 'ATO Backpack' models are designed to sit behind the aquarium, trading capacity for a form factor that fits the stand. Against all of this sits the 5-gallon bucket, the de facto default that purpose-built reservoirs are marketed against.

On redundancy, Reef Casa says most ATO systems carry a second, emergency water level sensor that cuts the pump if the primary fails — the same failure mode reservoir sizing hedges against. No page says how the presence of that backup, or of run-dry protection or a siphon break, should change the sizing decision.

Placement decides more than capacity does

Where the reservoir sits will override the number of days you calculated. Melev's Reef documented the problem plainly: sump water at 9 inches, top-off container holding water as high as 30 inches, and an active siphon that had to be prevented rather than ignored. A raised reservoir will keep feeding the tank with the pump off. Bulk Reef Supply's advice is a siphon break valve on the fill line, and the same article warns the opposite case — a reservoir on the cabinet floor needs the fill line terminating above the tank water line, or display water back-siphons into your top-off supply. Either fault empties a large reservoir faster than a small one, which is the uncomfortable part of buying capacity.

Sensor and pump siting are the other fixed constraints. Bulk Reef Supply puts the ATO pump at the bottom of the reservoir to reduce the risk of drawing air, and in a sump or all-in-one it puts the sensor in the return pump chamber, the only chamber whose level moves with evaporation. For installation directly in the display aquarium, it names the XP Aqua Sumpless ATO as the only suitable system.

Inland Seas Simplicity 10 — Best long-interval capacity

• 10-gallon volume, the largest dedicated ATO reservoir here, for the most days between refills
• Wide-mouth lid flips over into a spout, so filling doesn't need a separate funnel
• But: 20.5 x 16 x 22.5 in footprint and 22.5 in height won't clear many stand openings
• But: 14.6 lb empty, so it is not something you shuffle around once sited

Bashsea 5 — Best narrow footprint

• 5 in wide across a 15 x 16 in body, so it fits tall thin gaps in a stand
• Acrylic construction with a lid included
• But: White acrylic means no external view of the water level
• But: 5 gallons halves the refill interval compared with a 10-gallon reservoir

Marinecolor® Acryli Made — Best multi-compartment

• Three sealed 1500 mL rooms in one 4.5 L body for ATO plus dosing supplements
• Clear 3.8 mm acrylic with printed scale marks, so levels are visible at a glance
• But: 4.5 L total is very small for top-off duty — daily or near-daily refills on most tanks
• But: Splitting the volume three ways leaves roughly 1.5 L for ATO water

Useek — Best discreet sensor

• 0.7 in diameter sensor body disappears in rimless tanks and small sumps
• 1 mm sensing precision keeps salinity swings tight between pump cycles
• But: No reservoir supplied, so refill interval depends entirely on the container you pair it with
• But: ABS plastic rather than the acrylic used on the storage vessels here

EPTTECH — Best no-probe install

• Magnetic external mounting means nothing hangs inside the water
• Works through transparent walls up to 12.5–13 mm thick
• But: Will not work on opaque, painted-back or thicker-walled tanks or sumps
• But: No reservoir included, so the refill interval is whatever you supply

Useek — Best colour match

• Identical 0.7 in sensor diameter and 1 mm precision in a blue finish
• QST timeout protection covers overfilling, dry running and sensor failure
• But: Functionally the same unit as the green version — the only real difference is colour
• But: Reservoir not included

Back to the 400 gallon reef: which of these suits which case

Start from the refill interval. On that 400 gallon reef, Melev's Reef measured 3.6 gallons a day averaged over the first year, and the reservoir is fed directly by a 150gpd boosted RO/DI line. Plumbed refill changes the arithmetic entirely: capacity stops being about how often you haul jugs and becomes only a buffer against the RO/DI unit being offline. Nobody buying off a shelf has that. Reef Casa matches reservoir capacity to specific tanks in its own lineup rather than selling one size for everything.

For an evaporation-heavy reef with cabinet floor to spare, a 10 or 15 gallon reservoir buys the most days — Simplicity's 10 gallon Easy-Fill at $219.95, or Innovative Marine's 15 gallon Hydrofill at $264.00. For a narrow gap beside the sump, Reef Casa's slim and 'ATO Backpack' models trade capacity for a form factor that fits behind the tank. On a budget, Red Sea's 4.8 gallon ReefCan at $75.00 or Flipper's gravity-fed reservoir at $59.99. None of these listings publish dimensions, so measure the stand opening first.

I have not run any of these. This is published seller specification against a single hobbyist's measured evaporation — and no buyer or owner reports on how any given reservoir size works out in practice surfaced at all. Measure your own evaporation rate before buying.

Which one, then?

Inland Seas Simplicity 10, best for you want the longest possible stretch between refills on an evaporation-heavy reef tank and have sump-cabinet floor space to give it.

If your case is different:

• For you need a reservoir that slides into a narrow gap beside a sump rather than sitting in open floor space: Bashsea 5.
• For you want one container that handles top-off alongside two dosing liquids without three separate vessels in the cabinet: Marinecolor® Acryli Made.
• For you already have a container and need the sensing and pumping half of the system, in a tank where the sensor must be almost invisible: Useek.

The Inland Seas Simplicity 10’s weak spot: 20.5 x 16 x 22.5 in footprint and 22.5 in height won't clear many stand openings