Marine Aquarium Acronyms: FOWLR Tank Defined

FOWLR = Fish-Only-With-Live-Rock

FOWLR = Fish-Only-With-Live-Rock

Marine aquarium hobbyists do love their acronyms—SW, SG, SPS, LPS, BB, HO, VHO, DOC, LR, LFS, and RO/DI to list but a few. There are so many of them floating around out there on aquarium forums and in hobby literature that newcomers sometimes feel they need to learn a whole new language before they can join in the conversation.

One of the many marine aquarium acronyms getting tossed around these days (including right here at Saltwater Smarts) is FOWLR, as in “FOWLR tank.”

So who is this “Fowler” guy, and what’s so special about his aquarium?

The acronym FOWLR simply stands for Fish-Only-With-Live-Rock, and it’s pretty much just what the name implies—a marine aquarium that is aquascaped with live rock and stocked with only fish (though one could argue these systems might also contain motile invertebrates, such as cleaner shrimps, but I digress).

FOWLR systems have much to recommend them over your traditional fish-only (or FO—there’s another acronym!) system, which in years past would likely have been aquascaped with dry rocks and/or bleached coral skeletons. The benefits include:

Ease of cycling

The addition of quality live rock makes cycling a new tank a snap because it contains everything you need to establish biological filtration. Beneficial nitrifying bacteria are already present on the porous rocks, and the limited degree of additional die-off that commonly occurs after live rock is added to a new system provides just enough ammonia to fuel the growth of these bacteria.

Greatly enhanced biodiversity

The fish-only systems of yore (the time period just after the olden days but preceding yesteryear) simply cannot compare to today’s FOWLR systems with respect to biodiversity. Quality live rock is a veritable treasure trove of life that, if not already visible upon purchase, emerges in the days, weeks, months, and even years that follow.

In addition to beneficial bacteria, live rock can harbor an enormous variety of encrusting flora and fauna, including coralline algae, various forms of macroalgae, fan worms, bristleworms, amphipods and copepods, crabs, shrimps, snails, tunicates, brittle stars and sea stars, urchins, anemones, coral polyps, and much more. In essence, any rock-encrusting organism that occurs naturally in the ocean could potentially pop up in your aquarium.

A step away from reef systems

FOWLR systems represent a sort of middle ground between traditional fish-only systems and reef systems. To convert a FOWLR tank to a reef system, generally all you need to do is upgrade to reef-quality lighting. Everything else should already be in place.

Do be aware, however, that some fish and motile invertebrates that are suitable for a FOWLR tank might not be appropriate for systems containing corals and other sessile invertebrates. Be sure to research the species in your care to verify that they’re reef-safe before making the conversion.


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About Jeff Kurtz

Jeff Kurtz is the Co-founder/Editor of Saltwater Smarts, former Senior Consulting Editor for Tropical Fish Hobbyist Magazine, and the aquarist formerly known as “The Salt Creep.” He has been an aquarium hobbyist for over 30 years and is an avid scuba diver.

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