One of the most widely held myths in fishkeeping today is that brackish water aquaria cannot support plant life of any kind. Indeed, a lot of aquarists assume that plants aren't common in brackish water environments anyway, so the absence of them from aquaria is no big deal. The reality is that plants are very common in brackish waters, and in aquaria can be used very effectively to re-create specific habitats, to prevent the growth of unsightly algae, and to provide hiding places for fishes that don't like to swim out in the open all the time. Mangroves and seagrasses in particular define the environments that they occur in, and many aquarium fish naturally come from places where those are the dominant plant species. While seagrasses are still uncommon in the aquarium hobby, mangroves are increasingly widely sold, even though their is little information provided about them in the fishkeeping literature. This article will, in part, remedy this situation.
Besides mangroves and seagrasses, a variety of other plants occur in brackish water, including species widely traded for use in freshwater aquaria. These plants, including the versatile Java fern Microsorium pteropus, can be used in low-salinity brackish water tanks, making them invaluable additions to aquaria containing things mollies, bumblebee gobies, glassfish, figure-8 pufferfish, orange chromides, and all the other small aquarium fish that appreciate a little salt in their water. The plants also differ from the seagrasses and mangroves in being far less difficult to keep, and work well in much smaller aquaria than those larger species. Of these salt-tolerant freshwater plants, more will be said later, but to begin with, let's look at the mangroves and how they can be cultivated in the aquarium.
Mangroves were virtually absent from the hobby until relatively recently, but in the last few years they have become quite widely sold. They are sometimes advertised as highly efficient nitrate and phosphate removers for marine aquaria, the evidence for this is slight, especially given their very slow growth rate. If you want a nitrate removing plant, then fast-growing algae, like Caulerpa, or in freshwater tanks plants such as pondweed and hornwort, actually make much more sense than mangroves.
One of the most important facts to understand about mangroves is that they are trees. While they can be kept relatively small by careful pruning, even under the best of circumstances mangroves are much larger than can be easily accommodated in covered aquaria with lights built into the hood. To grow mangroves you need a tank with an open top and strong lights that are suspended well above the surface of the water. The second critical fact to appreciate is that mangroves are not aquatic plants but trees that have adapted to having their roots continually submerged (or at least buried in waterlogged mud). The leafy part of the mangrove plant is above the waterline most of the time, and only the seedlings are submerged during high tides. In short, mangroves are not really suitable for the vast majority of aquaria despite their relatively wide sale.
Mangroves are not especially difficult to care for provided you understand their basic needs. Firstly, all mangroves need the same level of illumination as marine invertebrates such as corals and anemones. Strong fluorescent tubes with reflectors, or else high-intensity lamps, will do the trick nicely. Secondly, mangroves need a deep, rich substrate. In the wild mangroves live rooted in thick and anoxic mud, but this can difficult to mimic in the aquarium without making a mess or poisoning the fish. Ideally, use a mix of laterite with river sand or fine gravel. Some species can be grown in plain coral sand or gravel, but regardless of the substrate used it is important to ensure that there is sufficient depth for the roots to develop properly. Even baby plants need a substrate depth of at least 10 cm, while more mature plants will need two or three times that amount to do well. Thirdly, mangroves need warmth, and while they can tolerate cool air for short periods, growth is best when the air temperature around them is kept to around 20Ã»C. The warm air rising from the aquarium should take care of this, provided there are no cold draughts blowing across the top of the tank. Finally, mangroves hate dry air, and most need to be exposed to air with a relative humidity of at least 50%. Again, the water vapour rising from the tank should keep the air around the mangrove nice and moist provided draughts are avoided. One thing that mangroves are largely indifferent to is salinity. Although often thought of marine plants, many species will be found in a variety of habitats from freshwater marshes to hypersaline lagoons with a higher salinity than the sea.
Mangroves are usually offered as either propagules or seedlings. Propagules resemble large bean pods, but are rounded at one end and tapering at the other. Propagules lack leaves and roots, and are the distributive phase of the mangrove life cycle, floating about on the tides until they are cast up on some empty beach where they can take root and develop into a mangrove tree. Seedlings are propagules that have started to develop, and will have roots and one or more leaves. Of the two, propagules are the best investment for a variety of reasons. For one thing, propagules of a variety of species can be purchased via mail order making it easier for aquarists to select the varieties most suitable for their needs. Seedlings, being more fragile, are more normally sold as specimen plants in aquarium shops. Propagules are also more tolerant of water chemistry changes, and can be planted into whatever conditions are required and expected to do well. Seedlings, by contrast, will be pre-adapted to a particular salinity, and will not react well to a different set of conditions. If placed in the 'wrong' salinity, seedlings will go into shock and lose all their leaves, and very often simply die. When buying seedlings from a shop, find out what salinity they are adapted to before spending any money. In fact, unless you are an experienced mangrove grower, avoid seedlings altogether and stick with propagules.
Growing mangrove propagules
There are basically two ways to 'sprout' propagules: stick them straight into the substrate as you would any other aquarium plant, or else float them in an aquarium until the roots and leaves begin to grow.
Floating is the best approach and essentially mimics the slow drift of the propagule along the coast before it finds its eventual home. Propagules float by themselves, so all you need to do is ensure that it doesn't bounce around the tank getting damaged or bumping into fish. The replacement suction cups sold in aquarium shops for use on heaters are ideal for this, particularly the ones with adjustable clips. The propagule can be clipped into one of the suction cups and then stuck to the side of the tank with the brownish blunt end is in the water and the tapered top two-thirds above the waterline. Eventually the propagule will sprout, usually the roots first and leave some time afterwards. As the roots grow the propagule can be raised slightly higher out of the water so that the roots remain submerged but the propagule does not. This causes the arched prop roots to grow as well as the central taproot, giving the mangrove its classic shape. After around six to twelve months the mangrove will be ready to be planted in the aquarium substrate. The dense tangle of roots provided by a few of these plants will form a very authentic and attractive home for most tropical brackish water fishes, but especially species like mudskippers and archerfish that are peculiarly adapted to life in and around mangrove swamps. A thicket of mangrove roots would also work rather nicely with fishes from forest streams as well, such as tetras and angelfish, where the shady spaces between the roots would enhance the colours of these types of fish.
Sticking the propagule into the substrate without this floating phase doesn't cause any practical problems, but the prop roots won't grow when the propagule is treated this way, so what you end up with is something more like a regular plant instead of the complex tangle of roots that make mangroves so distinctive. On the other hand, this approach is simple and easy to do: gently wedge a propagule into the substrate or attach it to some hard object like a piece of bogwood or tufa rock, and then let it grow. At this point in its life cycle the propagule can be fully submerged, but as it grows the leaves will eventually be raised above the waterline.
Whether or not the mangrove is grown with prop roots, it is best to put the plant into a ceramic pot and then bury that in the substrate. If the roots are buried directly in the substrate, you run the risk of these massive, woody roots cracking the sides of the tank or forcing the silicone sealant away from the glass. It is certainly possible for mangrove roots to 'break out' of an aquarium this way, which would obviously be disastrous for both fish and fishkeeper!
Once a mangrove is established, maintenance is very simple. Periodic spraying with freshwater is important, mimicking rainfall and washing away the salt that builds up on the leaves. Pruning is obviously essential if you want to keep a mangrove to a certain size, but this has to be done carefully, as too much or clumsy pruning can send a mangrove into shock or even kill it.
Seagrasses are unfortunately more easily bought in their dried state, used as a type of rattan in various Asian handicrafts, than as living plants. While some mail-order vendors in the United States are shipping live seagrasses, obtaining them elsewhere is significantly more difficult. Seagrasses are in many ways much better aquarium plants than mangroves. For a start, they are true aquatic plants and don't need any space above the waterline. Seagrasses also grow quite rapidly, and because they grow from the base of the plant, tolerate pruning well.
Seagrasses require a lot of light and need a deep, rich substrate. In terms of light, they are up there with corals and anemones, 5 Watts per gallon being considered the absolute minimum. This is quite a bit more than the average freshwater plant requires. Seagrasses normally grow in substrates that contain a mixture of mud and sand, and in the aquarium this is definitely something to be re-created. River or aragonitic sand mixed with mud or silt will provide the best substrate for most seagrasses. Plain coral sand or silica does not seem to work as well. The depth of the substrate is also important. The crown of the plant (the bit from which the roots and leaves grow) needs to be around 10 cm below the surface of the substrate, and the roots will need at least another 10 cm below the crown for them to grow and spread out normally. The depth of the water is relatively unimportant and will be defined by the species of seagrass used. Provide enough depth for the seagrass to grow more or less erect, rather than flopping across the top of the tank.
Planting seagrasses is essentially similar to planting Vallisneria: never ram the plant into the substrate, but instead dig a small hole, drop the plant in, and then let the sand roll back into the hole and cover the roots. Though they may shed some leaves after being transplanted (again, just like Vallisneria) once established, seagrasses aren't difficult to maintain. They grow quickly if they're happy, and need to be pruned back regularly. In the wild, things like turtles and manatees eat them, so the aquarist needs to 'play herbivore' to keep things in good order.
Salt-tolerant freshwater plants
Pretty much any plant that tolerates hard, alkaline water will do well in very slightly brackish water as well. Many of the commonly traded species of Vallisneria, Echinodorus, and Aponogeton, for example, will do well at SG 1.002-1.003, which is salty enough for many of the small gobies and livebearers that need brackish rather than freshwater conditions to be at their best.
There are some plants, however, which inhabit brackish in parts of their ranges, and these can be relied upon to prosper in slightly more saline conditions. Java ferns are particularly well known for this, and will easily adapt to SG 1.005, and some aquarists have had luck get good growth even at substantially higher salinities. Crinum calamistratum is another species that does well at SG 1.005, and with its beautifully ruffled leaves, adds a fantastic touch to any planted aquarium. Though not as frequently seen, Crinum pedunculatum also does well in slightly brackish water. Water hyssop or dwarf Bacopa, Bacopa monnieri, is another pretty plant, though a little demanding in terms of needing bright light. It is very common on coastal marshland where it usually grows at least partially emerse, forming dense thickets tipped with pretty white flowers. Finally, Cryptocoryne ciliata is a sturdy 'crypt' that naturally inhabits tidal areas even alongside species like mangroves. It is very tolerant of salty water, and will accept easily a specific gravity of up to SG 1.005, and possibly quite a bit higher. It does need good lighting and a laterite-enriched substrate though, and cannot be expected to grow in plain gravel or sand. Lilaeopsis brasiliensis and Samolus valerandi are two plants that will also thrive in brackish water, the latter having been known to tolerate salinities well above SG 1.010, but both are quite difficult to grow, needing a loamy substrate and very intense lighting.
Though aquarists are increasingly aware of the diversity of brackish water fishes and their fascinating biology, brackish water plants are still rather new to the hobby. Hopefully this will continue to improve as more and more aquarists become familiar with mangroves, seagrasses, and salt-tolerant plants such as Cryptocoryne ciliata. In the meantime, this article should have helped dispel any notion that brackish water fishes and aquarium plants don't mix!