Lake Malawi Biotope: Rocky Mbuna Shore and Sandy Zone
Lake Malawi biotope aquarium: hard alkaline water at pH 7.7–8.6, the rock zone of the mbuna versus the sand zone of Aulonocara, rock-pile aquascaping, overstocking and hybridisation risks.
The lake and its water
Lake Malawi (also called Lake Nyasa) is a rift lake shared by Malawi, Mozambique and Tanzania. According to Wikipedia it is 560–580 km long and up to 75 km wide, covers about 29,600 km², and reaches a maximum depth of 706 m with an average depth of 292 m. Oxygen is present only in the upper layer, down to roughly 250 m; below that the water is anoxic, so all fish life is concentrated near the surface and along the shores.
The water is warm, hard and alkaline, and unusually clear for a tropical lake. Wikipedia gives a typical surface temperature of 24–29 °C, about 22 °C in deep sections, a pH of 7.7–8.6 and visibility of up to 20 m. These values define the biotope: a Malawi aquarium is a hard-water, high-pH system, not a soft-water one. FishBase lists the zebra mbuna Maylandia zebra at pH 8.0, hardness 9–19 dH and 22–28 °C, and Seriously Fish gives 10–25 dGH and pH 7.6–8.6 for the same species, which is consistent with the lake data.
Rock zone and sand zone
The shoreline is divided into two contrasting habitats, and the fish fauna is divided with it. The rocky shore consists of boulders, in nature sometimes metres across, resting on fine sand (Practical Fishkeeping). It is the home of the mbuna, a name that means "rockfish" in the language of the Tonga people of Malawi (Wikipedia). FishBase describes M. zebra living on rocky substrate at 6–28 m depth, on sediment-free rock in shallow water and on sediment-covered rock somewhat deeper, where it combs loose material from the algal biocover.
The sandy and "intermediate" zones, where rock gives way to open sand with scattered stones, are occupied by different cichlids. Seriously Fish places Aulonocara stuartgranti in exactly these intermediate zones, feeding by sifting sand for invertebrates; Tropical Fish Hobbyist notes that Aulonocara locate buried prey with their lateral line in low light and darkness. Open water and sandy areas also hold the utaka, plankton-feeding cichlids of the open water, which Wikipedia contrasts with the rock-bound mbuna.
Typical genera
Wikipedia lists at least 700 cichlid species in the lake, with some estimates reaching 1,000, almost all of them endemic haplochromines. The mbuna genera named by Wikipedia are Abactochromis, Chindongo, Cyathochromis, Cynotilapia, Genyochromis, Gephyrochromis, Iodotropheus, Labeotropheus, Labidochromis, Maylandia, Melanochromis, Petrotilapia, Pseudotropheus and Tropheops. The sand and open-water fauna includes the peacock cichlids Aulonocara (at least 20 species according to TFH) and the utaka and other "hap" genera such as Rhamphochromis.
- Maylandia zebra — rock dweller, up to 11.3 cm SL, herbivorous grazer of the algal biocover; IUCN Least Concern (FishBase, 2018 assessment).
- Labidochromis caeruleus — rocky areas and Vallisneria beds in the north of the lake; the "electric yellow" form comes from the Charo–Lion's Cove area (Seriously Fish).
- Aulonocara stuartgranti — intermediate sand–rock zone along both coasts, benthic invertebrate feeder (Seriously Fish).
- Utaka and open-water haps — planktivores of sandy bays and open water, ecologically separate from the mbuna (Wikipedia).
Aquascaping with rock piles
The aquarium version of the rocky shore is a pile of rock on a sand base. Practical Fishkeeping recommends stacking rocks high to break lines of sight and to give each male a territory, with individual stones of roughly 15 cm, and up to about 30 cm across for a more habitat-correct look. Seriously Fish describes the same structure for L. caeruleus and M. zebra: piles of rock forming many caves and crevices with small areas of open water between them, over a sandy substrate.
Calcareous rock such as limestone buffers pH and hardness and is therefore suited to this biotope, whereas abrasive rock is unsuitable for fish that graze on surfaces (Practical Fishkeeping). Rooted plants are generally uprooted by mbuna (Wikipedia); the biotope is normally kept without them, although Seriously Fish notes that L. caeruleus also occurs in Vallisneria beds in nature. For a sand-zone tank, Seriously Fish and TFH describe scattered rockwork with open sand between the piles, and fine sand rather than coarse substrate so that sifting species can feed naturally.
Stocking philosophy
Mbuna are strongly territorial. Wikipedia notes that males hold roughly spherical territories and that overcrowding is used deliberately to spread aggression. Practical Fishkeeping puts this into numbers: a minimum of about 20 fish, preferably 30–40, in a tank of at least 120 cm, or 90–100 cm and 180 L for the smaller species, ideally started as similarly sized juveniles under 5 cm. Seriously Fish advises no more than one M. zebra male in tanks under 200 L, with three females per male, and harems of one male to three or more females for L. caeruleus.
Two rules follow from the biology. First, mbuna and Aulonocara are not natural tankmates: Seriously Fish and TFH both describe most mbuna as too aggressive for peacock cichlids, so a rock-zone tank and a sand-zone tank are best treated as separate biotopes. Second, hybridisation is a real risk. Seriously Fish warns that most Aulonocara species can interbreed and that different colour forms of L. caeruleus should not be mixed; Practical Fishkeeping adds that females should never be kept without a male of their own species.
Diet, water quality and water changes
Mbuna are grazers, and Seriously Fish states that vegetable matter should dominate the diet of M. zebra, with high-protein foods avoided. Practical Fishkeeping links frozen bloodworm to "Malawi bloat", a frequently fatal condition, and suggests one feeding-free day per week. Sand-zone Aulonocara take small invertebrates and fine-grade foods in small, frequent feedings (TFH, Seriously Fish).
Because the tank is heavily stocked, water quality has to be kept close to the lake's own: Wikipedia and Practical Fishkeeping both call for ammonia and nitrite at or near zero, and PFK sets nitrate below 40 ppm, achieved by strong mechanical and biological filtration, extra aeration and frequent partial water changes. A temperature of 24–26 °C and pH of 7.5 or higher are given as targets by PFK; reverse-osmosis water should not be used without remineralisation. All species named here are maternal mouthbrooders; females carry the brood for about three to four weeks (Seriously Fish, TFH).