Salty Q&A: What Constitutes a Reef Aquarium?

Here's a recent shot of my 125-gallon reef aquarium

Here’s a recent shot of my 125-gallon reef aquarium


I’m new to saltwater aquarium keeping and struggling to wrap my head around all the different ideas and terminology. For example, what exactly constitutes a reef tank versus a fish-only tank that happens to include a few invertebrates?” – Submitted by Brent M.


If you’d asked me to distinguish between these two aquarium types 20 years ago, I’d have a fairly straightforward answer. I’d tell you that a fish-only tank, as the name implies, contains only fish and possibly a few motile invertebrates while a reef aquarium (or “mini-reef,” as this type of system was known back then) puts the focus almost exclusively on corals and other sessile invertebrates, with any fish intentionally limited to small numbers and relatively diminutive species.

But since you’re asking this question in 2016, I’d have to say—and, fellow salties, correct me if I’m wrong here—that most marine aquarists don’t fit so neatly into the fish-only or reef aquarium “camps” anymore. Nowadays, the distinction seems to be blurring. While there are still hobbyists who limit their stocking schemes primarily to fish or invertebrates (and there’s absolutely nothing wrong with that), I think the majority of hobbyists today fall somewhere in the middle.

But is there a point at which such a hybrid marine aquarium can be officially designated a reef tank, some obvious line of demarcation between one and the other? I don’t believe so, and if you were to ask 10 people to identify this line, I suspect you’d get 10 different answers. You could probably get agreement at the margins—i.e., that a tank containing mostly fish and, say, just a few zoanthid or mushroom-polyp colonies is probably not a reef tank while a system brimming with Acropora and other stony corals probably is. However, where one type ends and the other begins is subject to interpretation.

My current 125-gallon tank comes to mind here. Many of the invertebrates it contains are free-standing gorgonians. Apart from a handful of lettuce corals, leather corals, and zoanthids, there aren’t all that many inverts actually attached to the rockwork. Plus, this tank contains several good-sized, energetic fish. Now, I call this a reef tank, but others might have a different take on the matter and it certainly doesn’t fit my earlier characterization of such systems.

When all is said and done, perhaps the best way to answer your question is: A reef is in the eye of the beholder.


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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.


  1. I think that a “Reef Tank” simulates a real reef in the sea. A Reef in the sea contains other things along with fish and anything that is in the sea could be put in a reef tank (with in reason)
    A fish only tank is an artificial environment because no where in the sea does this occur where there are just fish with nothing else. We use fish only tanks if we are trying to breed a certain fish or if it is a difficult fish to keep and we are trying different foods on it. If there was a swimming pool, and only Supermodels were allowed to swim in it, then it would be a Supermodel only tank, and those are rare.

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