How ReefRig decides

I’m an engineer planning my first reef tank, and I built ReefRig because I couldn’t find a buying tool that showed its work. So this page is the work. It explains exactly how the engine turns your tank’s dimensions and stocking plan into an equipment list, what each rule is protecting you from, and — most importantly — which published manufacturer numbers each rule reads, so you can re-derive any recommendation from the same spec sheets I did.

One thing this page is not: a review. ReefRig never says “we tested this.” I don’t run gear in a tank and rate it, and nothing here depends on my having done so. The claim is narrower and checkable — that published specifications, compared systematically against community-consensus sizing math, point to gear that fits. When a rule is built on softer consensus rather than a hard number, it’s labeled medium confidence and phrased as a recommendation rather than a hard check. Nothing is presented as more certain than its source.

The one number everything is sized from

Before any rule runs, the engine computes your effective system volume — the water the equipment actually has to service, which is never the number on the box. A display tank’s rated volume is reduced by about 15% for the space rock and sand displace. A sump adds roughly 70% of its rated volume (it runs below full, so there’s room to catch the drain-down when the return pump stops), and that figure is displacement-adjusted too. Every rule below sizes off this effective volume, not the advertised gallons. It reads two manufacturer numbers: the display tank’s rated volume and, on sump systems, the sump’s rated volume.

The eight sizing rules at a glance

Each rule states a sizing target as a band, not a single threshold — reef sizing lives in ranges, and a range is honest about it. For each one, here’s what it sizes, the beginner mistake it’s built to prevent, and the class of manufacturer spec it reads.

The rows below expand each rule. Where the reasoning rests on community consensus rather than a hard published number, the rule is flagged medium confidence.

R1 · Protein skimmer

A skimmer is sized from your effective system volume scaled by a bioload multiplier — 1.0 for a lightly stocked tank, 1.3 for medium, 1.6 for heavy — because more fish means more waste to export. The engine targets a rated processing volume of at least about 1.5× that bioload-adjusted figure, and it stops recommending skimmers so oversized that a lightly fed tank couldn’t give one enough organics to produce foam. On an all-in-one tank running soft corals or fish only, it skips the skimmer entirely: the rear chamber with filter media handles that nutrient load, and a skimmer is money better spent elsewhere. It reads the skimmer’s rated volume spec against your computed system volume. The trap it prevents is the most common skimmer mistake there is — buying one rated for your display gallons and finding it can’t keep up once the tank is stocked.

R1 also carries a hard fit constraint that most sizing charts ignore entirely, and it deserves its own section.

The depth trap: why a correctly-sized skimmer can still be wrong

A sump skimmer is only in-spec if the water around it sits inside the skimmer’s rated operating-depth range. Every in-sump skimmer publishes one — the band of water depth its pump was designed to draw air and produce a stable foam head in. Sit it in water that’s too deep and the pump is over-submerged; the foam collapses and the skimmer underperforms no matter how generously it’s rated. This is a genuine compatibility failure between two independently-correct specs, and it’s invisible on any “what size skimmer for X gallons” chart.

ReefRig checks it directly, comparing the skimmer’s published operating-depth range against the water depth of the tank’s sump skimmer chamber. If the chamber is deeper than the skimmer’s rated maximum by up to about three inches, the engine keeps the skimmer but flags it and calls for a 2–3" skimmer riser to lift the body back into its rated range. Deeper than that, or shallower than the rated minimum, and the pairing fails outright and the skimmer is dropped from the list.

A concrete, checkable case. A Reef Octopus Classic 150INT is rated to operate in 6–8 inches of water. The Red Sea Reefer 350 G3’s sump skimmer chamber runs about 9.2 inches deep — roughly an inch past the skimmer’s rated ceiling. A sizing chart would happily pair them; ReefRig flags the mismatch and adds a riser. The same trap appears one tier up: a Nyos Quantum 220, rated for 8 inches, in the Reefer 625 G3’s ~11-inch chamber. Both numbers come straight from the manufacturers’ spec sheets, which is the whole point — you can open the skimmer’s manual and the tank’s sump drawing and confirm the conflict yourself. Modern Red Sea Reefer sumps run deep enough that this bites a lot of otherwise-sensible skimmer choices, and it’s the finding that convinced me a sizing chart alone isn’t enough.

The engine applies the same footprint logic to the skimmer’s base: its published length and width must physically fit the sump chamber (in either orientation), or it isn’t offered.

R2 · Lighting

Corals are sized by the light they need at the sand, expressed as a PAR band per type: roughly 30–50 for a fish-only tank, 50–150 for soft corals, 100–200 for LPS, 200–400 for SPS, and 100–300 for a mixed reef. Fixture count is the tank length divided by a single fixture’s effective spread, rounded up; a front-to-back depth over about 24 inches triggers a note that you’ll want a second row or wider-spread fixtures to avoid dim back corners. It reads a fixture’s PAR and spread specs against your tank’s length and width. The trap: a light that looks plenty bright to your eye but never delivers the PAR the corals you actually bought need to color up and grow.

R3 · Flow

Flow is set as total in-tank turnover — display volume times a per-type multiple: about 5–10× for fish-only, 10–20× for softies, 20–40× for LPS, 40–60× for SPS, and 25–40× for a mixed reef. The engine prefers two opposed pumps over one large one, because random turbulence keeps detritus suspended far better than a single laminar jet, and it scales the suggested pump count with tank length. It reads a pump’s maximum flow spec against that turnover target. This one is medium confidence — flow needs are real but the exact multiple is community consensus, not a hard law — so it’s framed as a target range you dial in, not a pass/fail. The trap: dead spots where waste settles and cyano takes hold.

R4 · Heating

Heat is sized at roughly 3–5 watts per gallon of effective system volume. For any tank with stony corals, the engine splits that across two elements, each carrying about 60–75% of the load, so a single element failing on won’t have the wattage to cook the tank and a single element failing off won’t let it crash. And it always requires an external temperature controller: a heater’s built-in mechanical thermostat is the classic tank killer when it sticks closed. It reads heater wattage and whether your chosen controller can actually switch an AC heater. This is the one rule that will block a build — an uncontrolled single heater on a reef isn’t a preference, it’s a known way to lose the tank.

R5 · Return pump

A return pump is sized to move about 5× your system volume per hour through the sump — but the number that matters is flow at your actual head height, not the box rating. The engine estimates head loss from your vertical rise plus published plumbing coefficients (about a foot of loss per 90° elbow and a foot per ten feet of horizontal run), assuming a typical two-elbow return path on top of the rise you enter. It then reads the pump’s head curve — flow at height — rather than its wide-open maximum, and prefers a DC pump you can tune down to your overflow’s capacity. Doesn’t apply to all-in-one tanks, which use their stock rear-chamber pump. The trap: sizing off the box’s big flow number and getting a disappointing trickle four feet up.

R6 · Sump & ATO

Two things get sized here. Daily evaporation is estimated at 1–3% of system volume, and the auto top-off reservoir is sized to cover 7–12 days of it between refills — an ATO holds salinity steady far better than remembering to pour water in. On sump systems, the sump itself is sized at 20–40% of the display volume so it has the working room to swallow the drain-down when the return stops. It reads display and sump rated volumes. Medium confidence, since evaporation varies with your room and lid. The trap: salinity swinging with manual top-off, or a sump that floods the floor the first time the power cuts.

R7 · Dosing

Whether you need alkalinity dosing depends on what you keep. Fish-only and soft-coral tanks rarely consume alkalinity faster than water changes replace it, so the engine recommends no dosing gear at all. An LPS tank is told to monitor — test weekly, and add dosing only once daily consumption crosses about 0.45 dKH/day, not before. A stony-dominated or mixed reef is told to plan for it from the start, because that threshold usually arrives within months. It reads your coral type against a consumption trigger. This deliberately stops at chemistry — it sizes the dosing hardware and the trigger, and makes no claim about keeping any animal alive. The trap it prevents cuts both ways: buying pumps you won’t use for a year, or missing the moment water changes quietly stop keeping up.

R8 · Electrical load

The last rule is a rough steady-state power estimate — heater duty cycle, lighting, pumps, and a controller allowance, adjusted for how equipment-dense the reef type tends to be. Its job isn’t a precise watt count; it’s safety guidance: keep continuous load under 1,440 W per 15-amp circuit (the NEC 80% rule), run everything on GFCI, and put heaters on a different circuit than the return and wavemakers so a single trip never kills both heat and flow at once. Medium confidence by nature — it’s an estimate, stated as one. The trap: quietly overloading one outlet, or wiring the whole reef so a single fault takes it all down.

The compatibility layer: fit, not just size

Sizing tells you what a component needs to do; it doesn’t tell you whether two specific products work together. That’s a second layer. It picks parts by their spec attributes, not by brand names hardcoded into the engine, so the logic survives a product being renamed, revised, or discontinued:

• Controllers are chosen by capability — can this unit switch an AC heater, or is it a monitoring hub? — which then determines whether the heaters need their own inline controller. The heater and its control source are matched, never assumed.
• Battery backup (for high-end SPS builds, where a power cut suffocates corals within hours) is matched against its published pump-compatibility list. If the exact pumps in your build aren’t on it, that’s surfaced as a warning rather than a silent pass.
• Salt is chosen by its target audience attribute — a high-alkalinity mix for stony reefs, a balanced one for LPS, an economical one for fish or softies — not by name.
• Discontinued gear and anything without a current price is excluded outright. A part you can’t buy is never recommended, and if a whole slot has no valid in-stock option left, the gap is shown explicitly instead of quietly vanishing from your list.

The data behind the rules

Rules are only as trustworthy as the numbers they run on, so the data follows a stated discipline — the same discipline described in plainer terms on the About page, here in more technical detail.

• Every price carries a verification date. Reef gear pricing drifts constantly, so a number you can’t date is a number you can’t trust. Each product’s reference price is stored with the date it was checked; when it goes stale, it’s re-verified against the retailer, not guessed forward.
• Calculator constants were cross-checked one by one against official sources. The physical and chemical constants behind the calculators — salt-mix ratios, salinity-to-conductivity conversions, two-part dosing strengths, cleanup-crew densities — were each audited against manufacturer documentation and published references (ESV and BRS dosing specs, Kaye & Laby physical constants, Randy Holmes-Farley’s salinity work, and the salt makers’ own mixing rates). That audit is written up in the project’s launch records, anchor by anchor, with the source for each.
• The tools and the rules share one source of truth. Where a calculator needs a value the engine already defines — the heating math, the dosing trigger, effective system volume — it calls the engine’s function rather than copying the constant. There is no second, drifting copy of a rule to keep in sync.
• Discontinued products are flagged and benched. Once a product goes out of production it’s marked and kept out of every recommendation, so you’re never pointed at gear you can’t actually order.

What this doesn’t claim

ReefRig decides equipment, and only equipment. It won’t tell you how to treat a sick fish, dose medication, or keep a coral alive — that depends on judgment and observation a spec engine has no business faking, and keeping that line bright is part of how it stays honest about the thing it does cover. And to say it once more, because it’s the load-bearing distinction: none of this comes from running gear in a tank. It comes from comparing published specifications against stated sizing rules, consistently, with the sources attached — a smaller promise than “I tried them all,” and a more checkable one.

Found a rule that’s wrong, a spec I’ve misread, or a pairing the engine gets backwards? Tell me: hello@reefrig.com. Data only stays honest if the errors get caught, and a corrected rule makes the engine better for everyone who runs it after you.