Last Updated: September 28, 2026
The answer is a flow class, not a gallon count, so working out what size aquarium overflow box do you need comes down to one number: the actual return flow your pump delivers after head, which sets the minimum overflow capacity your particular tank actually requires.
As an Amazon Associate we earn from qualifying purchases.
By David Nguyen
Quick answer: Our top pick in 2026 is the Up to 300 — our #1 rated choice. See the full ranked comparison, alternatives and buying advice below.
How Much Overflow Capacity You Need
Sizing an overflow starts and ends with delivered flow, so ignore the tank’s gallon label for a moment. This section maps your actual after-head return flow to a minimum overflow rating, then weighs the configured cost, the maintenance conditions, and the failure or edge cases around that number.
Actual Delivered Return Flow in GPH
The primary axis is the flow your pump delivers after head, and the rule is simple: the overflow’s maximum rating must be at least your delivered flow. The lookup below turns that flow into a minimum class, from 300 GPH at the low end up to 2400 GPH for large systems.
| Actual delivered GPH | Minimum overflow rating | Example SKU | Conditions |
|---|---|---|---|
| Up to 300 | 300 GPH | Eshopps PF-300 | No drilling; siphon upkeep |
| 301–400 | 400 GPH | Fiji Cube EOF-4 | Drilled; template included |
| 401–600 | 600 GPH | Eshopps Eclipse S | Drilled; dual 1-in drains |
| 601–800 | 800 GPH | Eshopps PF-800 or Eclipse M | HOB or drilled |
| 801–1000 | 1000 GPH | Eshopps Eclipse L | Drilled; capacity is a ceiling |
| 1001–1200 | 1200 GPH | Fiji Cube EOF-12 | Drilled; Herbie/BeanAnimal-ready |
| 1201–1600 | 1600 GPH | Fiji LoPro 1600 | 3-drain; large system |
| 1601–2400 | 2400 GPH | Fiji LoPro 2400 | 3-drain; matching plumbing |
Read your delivered flow, then drop to the row that covers it. A tank delivering up to 300 GPH needs at least a 300-class box like the PF-300; 401 to 600 GPH calls for a 600-class Eclipse S; and 1601 to 2400 GPH points to a LoPro 2400, each an example rather than the only option.
Configured Cost
Cost climbs with the class, roughly tracking capacity. The 300-to-800 GPH boxes here run about $119.98 to $159.98 as of 09/2026, while the larger examples reach further: the Fiji EOF-12 near $157.24 on sale and the LoPro 2400 around $254.99 on sale for big systems.
Do not overbuy the class to feel safe. A larger box costs more and adds no benefit if your delivered flow does not reach it, so size to the row your real flow lands in, treating any step up as a deliberate choice for planned future flow rather than a default hedge.
Maintenance Conditions
Within a class, architecture changes upkeep. The same 800-GPH rating covers the hang-on PF-800, which needs siphon maintenance, and the drilled Eclipse M, which runs on gravity once tuned, so a size class can contain very different maintenance profiles at the same flow number.
Factor that in when the lookup offers options. If two boxes cover your flow, the drilled one usually asks less ongoing attention while the hang-on avoids drilling, so let maintenance tolerance break the tie once the class is set by your delivered flow rather than the other way around.
Failure and Edge Cases
The main edge case is head miscalculation. A worked example — four feet of vertical lift, four feet of horizontal run, and two ninety-degree elbows — totals roughly 6.4 feet of head, where a pump might deliver about 700 GPH, pointing to an 800-GPH class rather than a smaller one.
The other trap is a phantom safety factor. No universal twenty or twenty-five percent reserve above delivered flow was verified, so do not pad the class by a fixed percentage; read the pump curve at your real head and size to that figure, adding headroom only by deliberate judgment.
Adjust for Your Situation
Once the lookup gives a class, these four common boxes show how the smaller ranges map to real products. Each notes its verified specs, a relative price tier, and the drawback to weigh, so you can adjust the class for architecture and fit rather than reading tank gallons alone.
Eshopps PF-300
The Eshopps PF-300 anchors the smallest class, a 300-GPH hang-on U-tube for delivered flows up to about 300 GPH on tanks to roughly 75 gallons. If your pump curve at head lands in that range, this is the minimum class, and needing no drilling is the situational bonus.
Around $119.98 retail or $129.98 from the maker as of 09/2026, adjust away from it if your delivered flow exceeds 300 GPH, since it would then be undersized. Its siphon upkeep is the cost of the no-drill convenience, so weigh that against a drilled box in the same low range.
Fiji Cube EOF-4
The Fiji Cube EOF-4 covers the 301-to-400 GPH class, a drilled low-profile external overflow with a template and diamond holesaw. When your delivered flow steps just past the smallest class and the panel is drillable, it is the natural next size with install hardware included.
At about $118.99 on sale or $139.99 regular as of 09/2026, adjust to it when your flow clears 300 but stays under 400 GPH. Drilling is required, so if the glass cannot take a hole, jump the situation to a hang-on class instead rather than forcing this drilled box onto the tank.
Eshopps Eclipse S
The Eshopps Eclipse S fills the 401-to-600 GPH class, a drilled box with dual one-inch drains. For a delivered flow in that band on a drillable tank, it provides the capacity and the redundancy the smaller classes lack, adjusting the situation toward a more robust sump feed.
At about $139.98 retail or $149.90 from the maker as of 09/2026, it needs a drillable panel. If your flow lands here but you cannot drill, adjust to an 800-class hang-on like the PF-800 instead, since matching the class matters more than the specific architecture in the lookup.
Eshopps PF-800
The Eshopps PF-800 anchors the 601-to-800 GPH class without drilling, an 800-GPH hang-on U-tube for 75 to 125 gallon tanks. When your delivered flow is high but the glass is off-limits, it covers the upper common range that would otherwise force a drilled box.
Around $129.98 retail or $139.98 from the maker as of 09/2026, its 800-GPH maximum is not forced sump flow, so it is a ceiling, not a target. Adjust upward to a 1000-class Eclipse L only if your delivered flow genuinely exceeds 800, not merely to feel a margin of safety.
Frequently Asked Questions
Buyers sizing an overflow ask a consistent set of questions, usually starting from tank gallons. Each answer opens with the practical call, then the condition that changes it, so you can size from delivered flow, the number that actually decides the class you need to buy.
What spec should decide this purchase?
Actual delivered return flow in GPH should decide it, full stop. Tank volume can estimate a desired return flow, but it does not directly select the overflow, so read your pump curve at your real head and match a box rated at or above that delivered figure.
Use the lookup to convert flow to a class, then adjust for architecture and fit. Two tanks of the same gallons can need different classes if their pumps and plumbing differ, which is exactly why the flow number, not the gallon count, sits at the center of the sizing decision.
Which numbers are hard facts and which are heuristics?
The GPH ratings, the lookup thresholds and the September 2026 prices are hard facts, and the direct rule that rating must clear delivered flow is firm. The head example — about 6.4 feet from four feet of lift, a horizontal run and two elbows — is a worked estimate, not your exact figure.
The turnover multipliers and any reserve are heuristics. Two-to-five-times turnover gives a planning range, such as 110 to 275 GPH for a 55, but the pump curve gives the real delivered flow, and no universal twenty or twenty-five percent safety factor above it was verified.
Is the highest headline number always better?
No. A bigger class than your flow needs costs more and adds no benefit, since the overflow only passes what the pump delivers. Ratings measured under ideal conditions do not translate directly to your plumbing, so the right class is the smallest one that clears your delivered flow.
Size to the row your flow lands in, not the largest box you can afford. A 1200-class overflow on a 400-GPH return is wasted capacity and cost, so let delivered flow set the class and treat any step up as a deliberate choice for planned future flow, not a default hedge.
When might the reader not need this?
If your tank runs without a sump, there is no delivered return flow to size against and no overflow needed. A box exists only to feed a sump, so confirm that plan before working the lookup, since without a sump the sizing question simply does not apply to the tank.
And if you have not chosen a return pump yet, size the pump first, then the overflow to its delivered flow. The class follows the flow, so pin down the pump and plumbing before reading the table, ensuring the box matches the water the finished system will actually move.
Final Thoughts
In short, what size aquarium overflow box do you need is answered by one number: your delivered return flow after head, matched to a box rated at or above it. Read the pump curve, drop to the class in the lookup, and adjust for architecture and fit, since tank gallons estimate flow but never select the overflow.
Ready to decide? Our #1 pick for 2026 is the Up to 300.
Live price & availability on Amazon.
Related Reviews & Guides
11 Best Aquarium Wave Makers: Top Circulation Pumps for Reef and Planted Tanks
Aquarium Auto Top Off (ATO) System
