Here’s a recent shot of my 125-gallon reef aquariumQuestionI’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. Answer 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.