Setup guide

55 Gallon Reef Tank: Setup, Gear List & the Thing the Sale Price Doesn't Mention

The 55 is the tank everyone almost buys — it's four feet long, it's always on sale, and the number sounds substantial. Here's the honest version other guides skip: the 55 is only 13 inches front-to-back, and that one dimension is why so many reefers regret it. We'll show you when it still makes sense, and exactly how to build one well.

Paid links: if you buy through links on this page we may earn a commission, at no extra cost to you — it never affects our ratings. How we make money.

The 13-inch problem, stated plainly

A reef tank is aquascaped front-to-back: rock structure, coral growth room, and open sand in front. At 48 × 13 × 21 inches, the 55 has less front-to-back room than a 40 breeder (18″) despite holding more water. Once the rock is in, corals grow toward the glass fast, big-bodied fish struggle to turn, and there's nowhere to put a powerhead that isn't blasting something. The extra length vs. a 40B is real — but almost everyone who reefs a 55 wishes those gallons had been shaped like a 75 (same length, 5 more inches of depth, usually ~$100–150 more with the stand).

When a 55 still makes sense

Three honest cases: the tank is free or nearly free (hand-me-downs and $1/gallon sales are how most 55s happen); the stand location is genuinely too shallow for an 18-inch tank; or you're building a fish-and-softies display where wall-to-wall rockwork matters less. In those cases — build it well, and the length is still a real asset for wrasses and other cruisers.

Know your real volume first

A "55" typically sees water volume in the mid-40s after sand, rock, and the air gap. Pin down your true number before you dose anything: → get your true water volume (the 55 is a preset).

The core gear list

Sump + return pump

Most 55s aren't drilled, so you'll either run an overflow box to a 20-long sump or skip the sump and accept a hang-on-back skimmer (a real option at this size — less capable, much simpler). If you do run a sump, aim for roughly 7× display turnover delivered — about 385 GPH at the tank:

→ Size your return pump with the calculator

See our full return pump rankings →

Protein skimmer

The 13-inch width also squeezes the stand — check skimmer footprint against your sump before buying, not after. Rated-gallons numbers assume light stocking; derate them for a normal fish load:

→ Find the right skimmer for your bioload

See our full protein skimmer rankings →

Lighting

Same four-foot rule as the 75: two fixtures for even coverage. The good news is the narrow, 21-inch-tall water column is easy to light — mid-range fixtures reach the sand without breaking a sweat. Buy from the four fixtures with verifiable PAR data, not the marketing copy.

Flow

Target 20–40× total turnover (1,100–2,200 GPH), but placement matters more here than in any other size — in 13 inches of width, a powerful gyre or wide-flow pump pointed wrong is a sandstorm. Two smaller powerheads angled along the length beat one big unit. Work out your target →

Heat, rock, sand

200–250W of heat on an external controller (two smaller heaters if your sump has room — size it here). Rock: 40–55 lb, scaped tall and narrow rather than deep. Sand: a 1–1.5″ bed over the 48×13 footprint runs about 45–65 lb.

What a 55 build actually costs

Note what the table shows: the equipment costs almost the same as a 75 — four feet of lighting is four feet of lighting — so the "cheap" tank saves you $100–200 on glass and nothing else. That's the honest economics of the 55. Itemize your own build with the cost calculator.

Ranges, not quotes. If you're pricing from zero and can spend the extra ~$150, run these numbers against the 75 guide before committing — the delta buys 5 inches of depth you'll use every day.

Stocking a 55

Think length fish, not bulk fish: fairy and flasher wrasses, a midas blenny, clown pair, royal gramma, firefish, a goby/pistol pair. Skip tangs other than perhaps a bristletooth, and skip anything that turns like a truck — the 13-inch width is the constraint, not the 55 gallons.

The order of operations

Set the tank (overflow box or drill before water!) → plumb the sump → RODI, saltwater, aquascape → pumps, skimmer, heat on → cycle (2–6 weeks) → cleanup crew → fish, then corals, slowly.

Other sizes

The 40 breeder — less water, better shape, cheaper lighting (one fixture). The 75 — the same length done right. The 30 — the smallest size we'd call forgiving.