Reading Aquarium Specs
Four figures appear on most aquarium equipment listings, and every one of them is a narrower claim than it looks. This guide says what each is actually a statement about — and what it is silent on.
“For tanks up to X gallons”
This is the manufacturer’s rating, and it assumes conditions the listing does not state. For a heater it assumes a temperature lift; for a pump it assumes a tank shape and an aquascape that does not obstruct flow.
Treat it as a starting bracket rather than a fit. A heater rated for 30 gallons in a warm room is not the same heater in a cold garage, and rated volume says nothing about the lift it was rated against.
Flow rate in GPH or L/h
Almost always measured with no head height, no plumbing and no obstruction — an open pump on a bench. Real installed flow is lower, sometimes considerably.
More importantly, flow only becomes meaningful divided by your tank volume. The resulting turnover figure is what your livestock experience, and the same pump gives wildly different turnover in different tanks.
Wattage
This is the figure that most often means two different things depending on the product.
| On a… | Wattage means |
|---|---|
| Heater | Heat output — the rate it can deliver energy to the water |
| Pump | Power consumed — a running cost, and heat added to the tank |
| Timer or controller | Switched-load capacity — the maximum it can carry, not what it uses |
These are not comparable to each other, and pooling them is exactly the mistake that made the lighting-timer category unpublishable on this site: a light’s 20 W draw and a timer’s 1,875 W capacity in one column produce a median that describes neither.
Dosing rate in ml/min
A maximum, not an operating point. A doser rated at 100 ml/min asked to deliver 5 ml is working at the least precise end of its range, which is why a lower-rated pump with fine control is usually the better dosing instrument.
The general rule. Every one of these figures is a statement made under conditions the manufacturer chose and rarely publishes. That does not make them useless — they are comparable to each other within a category, which is exactly what our comparison pages use them for. It makes them a poor basis for predicting behavior in your specific tank.
What is almost never published
Accuracy and tolerance. Failure behavior. Noise. Service life and consumable replacement intervals. Calibration requirements. These decide how equipment behaves over years and in failure, and they are absent from structured data across every category on this site — which is why our pages publish distributions and decline to rank reliability.
Where the specifications run out
Coverage across this corpus splits the categories almost in half. Heaters state a wattage and a tank rating nearly universally; wave makers state a flow rate. Leak detectors publish a power figure on one listing in sixty, and water quality monitors on two. That is not an oversight this site can repair — it reflects how each category is sold, and a category described in features rather than figures is one where the decision has to be made on mechanism and reputation instead.
Tank ratings assume a tank you do not have
A stated tank rating assumes a room temperature, a lid, and a tank of ordinary proportions. A tall narrow tank in a cold room needs more heating than a wide shallow one of the same volume in a warm one, and the rating knows about neither. These figures are measured under conditions that make comparison possible, which is what they are for. Reading them as predictions about your tank rather than as a way of ranking products is where they mislead.
Why medians rather than averages
A single mis-typed specification moves an average and barely moves a median, and listings contain enough errors for that to matter. Class figures here also exclude values far outside the normal range for their class, so one listing with a decimal point in the wrong place cannot shift the number everything else is measured against. It is a duller statistic and a considerably more honest one.
A missing figure is information
A blank in a comparison table here means the listing did not publish the figure, and that absence is worth reading rather than skipping. A category where a specification is routinely missing is one where manufacturers decided it was not a selling point, and that decision usually reveals something. Nobody publishes probe drift rates. Nobody publishes feeder portion tolerances. In both cases the omission describes the product as clearly as a number would.
What this site does not do
Nothing here has been tested, run for a year, or measured independently. These pages read what manufacturers publish, normalise it so the same specification means the same thing between products, and compare it. That makes the comparisons fair and it does not make them complete: a listing can be wrong, and a manufacturer can revise a specification quietly. Where a figure looks implausible against its class median, the median is usually the more reliable of the two.
Where this site and a manufacturer disagree
Manufacturer copy describes what a product does under conditions chosen to show it well, and these pages describe what the specifications say across a whole category. The two diverge most where a figure is a maximum rather than a typical value — a flow rate with no head height, a tank rating in a warm room, a wattage at full output. Where the two disagree, the listing is usually right about that product and this site is usually right about what the category typically delivers.
Common questions
Is rated tank volume useless then?
No — it is a useful bracket and a genuine comparison between products. It is just not a promise about your room or your aquascape.
Why does wattage mean different things?
Because it is a unit, not a specification. Watts in describes consumption, watts out describes output, and watts rated describes capacity.
Should I ignore flow ratings?
Compare them to each other, then divide by your volume to get turnover. Do not read the raw number as what you will get in the tank.
Where do I find accuracy figures?
The manufacturer’s own documentation, if anywhere. They are not in the retail listing data this site is built from.