In a typical reef tank of yesteryear, the so-called closed loop systems played the role of wave making devices. With a reef tank, powerheads not only keep the rocks clean of ammonia-producing detritus while giving corals a chance to catch their dinner, but most importantly, they create water movement that is essential for gas exchange in the environment, in turn providing corals with oxygen to respire. In my planted tank days, I didn’t even own a powerhead until much later (a single Koralia as in the picture below), all my flow needs were met by duo canister filters, their outlets located at the opposite rear corners of the 75g, plant infested underwater jungle. I remember coming from a freshwater background into reefing and being amazed at how much flow was needed to keep corals alive, compared to the planted tank I used to own. Some corals like more, some less, but all need the “artificial” current a propeller pump creates in order to survive. You simply can’t keep a reef-dwelling coral for long if you don’t provide it with enough oxygen transporting flow. If you don’t, well, you better get one (or two, or more…) because there is no reef tank on this planet that looks good without proper water circulation. If you happen to be the owner of a reef aquarium hosting live corals, there’s a good chance you also own a powerhead.