The Marine Planted Tank: Creating Your Ocean Garden

You’ve successfully kept saltwater fish. Then you moved on to corals. Then, maybe you took on the slightly greater challenge of tridacnid clams or some other delicate or specialized species—with yet more success! What’s next?

There are hundreds of thousands of animal species that inhabit our planet’s tropical coral reefs. Even the most advanced reef aquarist could find novel challenges—new reef animals to keep—until the end of his or her days. But why not go for something entirely different? Like, plants?

Another World

The marine aquarium hobby has exploded in the last few years. The advent of social media, aquarium-related blogs and podcasts, a surging attendance at trade shows, etc. have greatly accelerated these changes. Our aquaria were once a place of personal refuge; now, for some of us, it might seem like the whole world is peering into them.

This certainly isn’t all bad. Most of us really do like the opportunity to share the many triumphs and tragedies that occur within those four glass panels. However, while this connectedness brings a greater sense of community to the hobby, it may also cause us to get bored with our tanks pretty quickly—especially when it’s all too obvious that we basically like and keep all the same stuff that the next guy does. Hence, the longing for radical change. A few veteran aquarists have discovered this “something different” in planted marine aquaria.

To clear from the onset: Most marine “plants” are actually macroalgae rather than true plants (Kingdom Plantae). Like many others, we label these algal “macrophytes” as such because they share many physical (and therefore aesthetic) characteristics. As we will see, although marine planted tanks are reminiscent of reef aquaria or freshwater planted aquaria, they are quite distinctive in maintenance and function.

 

The Plants

Because there are so few true plants in the marine environment, few are represented in the aquarium trade. They are nevertheless so ecologically important and so beautiful in appearance that they are worthy of mention. Of these, hobbyists are most likely to encounter mangroves and seagrasses.

The most commonly traded mangrove is Rhizophora mangle. It is similar in care to many other mangroves in that it requires a huge grow-out area (or heavy, frequent trimming as it grows). Mangroves prefer a relatively deep, fine-graded sand or mud substrate. Perhaps just as important is a healthy microbial sediment community that not only includes the usual players (e.g. nitrifying bacteria) but also rhizobial bacteria such as purple non-sulfur bacteria. Finally, but critically, mangroves require very intense lighting; this must be over the plant, which will of course grout out of the tank, so an additional light fixture is necessary where direct natural illumination is not abundant.

Seagrasses are similar to mangroves in their need for both bright light and a deep, rich, fine (even silty) substrate. As your micro-meadow develops, it forms a tight mat over the substrate surface. The blades of this plant often host a variety of epiphytes (plants that live on plants); while natural and interesting, heavy use of herbivores and perhaps UV sterilization becomes necessary is the plants become threatened by smothering. While seagrasses are, as a group, not easily found in the trade, the turtle grass Thallasia testudinium is available on occasion.

Whereas true plants are mostly restricted to freshwater and terrestrial environments, the macroalgae are almost all found in seawater. This huge group includes the Rhodophyta (red), Phaeophyta (brown) and Chlorophyta (green) macroalgae. It is important to note that these forms are lumped together based solely upon their plantlike appearance rather than ancestral lineage, as multicellularity (the trait that makes their complex “macro” morphology possible) evolved within the red, brown and green algae independently.

 

The Tank

The macroalgae differ from true plants in some notable ways. As most planted marine tanks will be dominated by the former, these differences will be reflected strongly in the way the system is built and cared for.

Unlike true plants which have roots, macroalgae secure themselves to the seafloor with holdfasts. Holdfasts most often attach to the surface of, rather than deeply penetrate, the substrate. Therefore, in the complete absence of mangroves and seagrasses, a marine planted aquarium can be entirely substrate-free (that’s not to say that you couldn’t add some sand for aesthetic purposes). For the most part, macroalgae-dominated tanks should be very rocky.

Water quality management will be very strange to someone who is well-accustomed to reefkeeping. For one, you could actually run out of, and need to add, key nutrients such as nitrate or phosphate! Regular replenishment of essential trace elements (iodine, iron, manganese and many others) is advised here, as macros tend to mop them up like sponges. On the other hand, macros can release plenty of waste products, particularly when grown in huge masses. As these are typically organic exudates, tactics for removal may include chemical filtration (e.g. activated carbon); however, because many chemical filtrants are capable of removing useful substances, protein skimming or biological methods (e.g. heterotrophic bacteria such as those in PNS Probio™) are recommended.

Though especially abundant on seagrasses, epiphytes can grow on (and harm) many seaweeds, particularly the ornamental types which are often slower-growing. Being softer than seagrasses and more likely to be eaten by creatures such as snails, the best option for herbivores in macroalgae-dominated tanks is a multispecies benthic copepod product such as Poseidon’s  Feast.

Lastly, a planted tank of any kind of course needs bright light. Lighting requirements vary a bit from species to species; even so, because macroalage are somewhat adaptable, full-spectrum, intense lighting will grow nearly all types well. All-in-all, with strong illumination and a bit of fertilization, a planted marine aquarium can provide a very out-of-the-ordinary experience with reasonably low effort!