Acorpora grown in aquarium under direct, high-flow conditions. The coral in the foreground was grown from a one-inch frag with a Tunze 6045 placed five inches away.
It’s often repeated that while most corals appreciate flow, very few of them appreciate direct flow. As with so many of this hobby’s often repeated bits of advice, this one should come with a few caveats.
The type of flow preferred by a coral depends almost entirely on which coral we’re talking about, or, more specifically, which region of the reef it (or its ocean-bound relatives) calls home. For the purposes of this post, let’s focus our attention on the reef crest and outer reef slope regions and the most flow-loving of corals, the stonies like Acropora, Porites, Montipora and others.
The corals of the outer reef slopes can handle extreme amounts of current. It’s safe to say that the average hobbyist’s SPS tank does not come close to matching the amount of flow found in these corals’ natural environment. Of course, it would be unrealistic to expect them to be equal. In aquariums we have many other factors to consider, such as the objections our sandbeds might raise and the fact that aquariums often house corals from other, lower flow regions of the reef. Aquariums are, almost always, a compromise.
But you don’t need to compromise on the quality of flow you provide. Because aquarium circulation is not just about quantity; quality is far more important. The bit of advice we opened this post with originated from a point in time when most aquarists were using impeller driven pumps. Better suited as in-line circulation pumps, this style has been pretty much entirely replaced for in-tank circulation by propeller driven powerheads such as our Stream and Nanostream models.
Tunze was one of the first companies to use propeller driven pumps and was the first to popularize their use in aquariums. The main advantage of this style of pump is that it produces a very wide Stream of water. This wider, slower moving stream induces movement in the water around it and this creates many secondary eddies and currents.† The resulting body of moving water is far superior to the direct, narrow flow of impeller-driven pumps. It’s also much closer to the natural movement of water found on reef crests and slopes.
In the right applications, Stream and Nanostream pumps can be used to provide direct flow. Depending on the size of the tank, size of the pump and the coral in question, the pumps can sometimes be placed closer than 6 inches from, and aimed directly at, the coral. Ideally, controllable pumps would be used and the flow would fluctuate. This fluctuation gives you a larger safety net, and can result in a more efficient use of electricity, but it is not entirely necessary.
Remember that as sessile creatures corals cannot move about their environment; they count on their environment to move about them. It brings to them oxygen and nutrients and clears away the wastes they produce. Aquarists employing the more aggressive pump placement strategy described above enjoy many benefits. Faster growth is one and more disease-resistant, more resilient, corals is another. It’s been shown that corals fare much better under certain stressors, such as the stress of high temperatures, if flow around them is increased.‡
One benefit which cannot be obtained by any other means is the more natural growth form seen when high-flow corals are grown in high-flow conditions. Instead of the gangly, open branched form Acroporas reluctantly take in lower flow aquariums, you’ll have happier colonies with tighter, denser, more natural branching. Happier corals make for happier aquarists… that’s a win-win situation, no compromise needed!
† This induction of water movement created by a stream of moving water is one of the reasons for which GPH is an oversimplified metric for evaluating aquarium powerheads.
‡ T. Nakamura, R. van Woesik, 2001. ABSTRACT: In the western Pacific during 1998, coral bleaching, or the paling of corals through loss of pigmentation or loss of symbiotic algae (zooxanthellae), coincided with some of the warmest sea-surface temperatures (SSTs) on record. However, there was considerable spatial variation in coral survivorship; for example, corals of the same species at different locations around the Ryukyu Islands (Japan), within kilometers of each other, showed vastly different responses. Some locations experienced 100% coral mortality while other locations, nearby, suffered little coral mortality. Here we show experimental evidence for high survivorship of Acropora digitifera coral colonies that were subjected to both high SSTs (ranging from 26.22 to 33.65°C) and high-water flow (50 to 70 cm s–1), while corals that were subjected to both high SSTs and low-water flow (2 to 3 cm s–1) showed low survivorship. All experiments were conducted under high irradiance (~95% photosynthetically active radiation). We also empirically show that no coral mortality occurred when SSTs were below 30°C (ranging from 26.64 to 29.74°C) under similar flow regimes. The spatial differences in coral mortality during the 1998 bleaching event may have been, in part, a result of differences in water-flow rates that induced differential rates of passive diffusion, which varied among habitats.