Indian Monsoon Stream Biotope Aquarium

Wet and dry regime of Indian subcontinent streams, Western Ghats and Ganges–Brahmaputra fauna (barbs, loaches, Badis, Dario), rock and leaf layouts and a framework for seasonal change.

The monsoon regime

The southwest monsoon generally begins between the end of May and the beginning of June and fades between the end of September and the start of October; it accounts for nearly 80% of India's rainfall, and some parts of the subcontinent receive up to 10,000 mm (390 in) a year. In the Western Ghats, where moisture-laden winds from the Arabian Sea are forced upward, annual rainfall averages 250 cm (98 in) and ranges from 100 cm (39 in) to 900 cm (350 in). The many small streams draining the range carry a large volume of water during the monsoon months. A secondary northeast monsoon follows from about October to December and provides 50–60% of the rain in Tamil Nadu.

The remainder of the year is comparatively dry. Loach species such as Botia almorhae are described by Seriously Fish as congregating in pools and stiller areas over rocky substrates and undertaking upstream migrations before spawning. A monsoon-stream biotope therefore represents a habitat that alternates between fast, high, often turbid water and a low-water phase of clear pools, exposed rock and accumulated leaf litter.

Two habitat types, one subcontinent

The Western Ghats are a UNESCO World Heritage Site (2012) and hold nearly 30% of India's flora and fauna, with more than 200 freshwater fish species, of which 35 also occur in brackish or marine water. Thirteen fish genera are endemic to the range. The barb genera Dawkinsia and Pethia were both separated from Puntius in 2012; Dawkinsia (17 species) is centred on the Western Ghats, while Pethia (about 46 species) ranges across South and mainland Southeast Asia.

The Ganges–Brahmaputra lowlands to the north and east are the home of the Badidae. Badis badis occurs in the Ganges, Brahmaputra, Mahanadi and Indus basins in ponds, rivers, ditches and swamps; its documented locality is a wide, shallow, slow-moving and moderately turbid stream over mud, often associated with water-lily beds. Dario dario is restricted to tributaries of the Brahmaputra in West Bengal and Assam, possibly reaching Bhutan, in shallow, clear streams over sand or gravel with dense growths of Hygrophila, Limnophila, Ottelia, Rotala and Vallisneria. Pethia conchonius bridges both zones, recorded from Pakistan, India, Nepal and Bangladesh in flowing hill streams, lakes and swamps up to about 1490 m above sea level.

Representative fauna

• Sahyadria denisonii — Western Ghats endemic (Kerala, Karnataka) of pristine, highly oxygenated headwaters and rocky pools with dense riparian vegetation; to 110 mm SL; 15–25 °C, pH 6.5–7.8, 5–25 dGH; groups of 6–10; IUCN Red List: Endangered.
• Dawkinsia filamentosa — Kerala, Tamil Nadu and Karnataka, most common on lowland coastal floodplains in fresh and brackish water; to 120 mm SL; 20–25 °C, pH 6.0–7.0, 2–15 dGH; schooling, 8–10 specimens; egg-scattering spawner.
• Pethia conchonius — to 100 mm SL (Seriously Fish) or 14 cm TL (FishBase); Seriously Fish gives 16–24 °C, pH 6.0–8.0, 5–20 dGH and FishBase 18–22 °C, pH 6.0–8.0, 5–19 dH; FishBase notes it tolerates low water temperatures; groups of 8–10; IUCN: Least Concern.
• Badis badis — to 60 mm SL; 15–25 °C, pH 6.0–7.5, 3–15 dGH; micropredator that often refuses dried food; cave spawner laying 30–100 eggs that hatch in 2–3 days; IUCN: Least Concern.
• Dario dario — to 20 mm SL; 18–26 °C, pH 6.5–8.5, 1–15 dGH; FishBase records about 26 °C in the wild; micropredator; rival males are highly aggressive, so single fish or pairs are kept; IUCN: Data Deficient.
• Botia dario — Ganges and Brahmaputra drainages of India, Bangladesh and Bhutan; to 130 mm SL; 23–26 °C, pH 6.0–7.5, 1–10 dGH; gregarious, at least 5–6 and preferably 10 or more.
• Botia almorhae — Ganges drainage; to 160 mm SL; 19–27.5 °C, pH 6.0–7.5, 2–12 dGH; needs well-oxygenated water with a degree of flow.

The parameter windows overlap only partly. A Western Ghats hill-stream tank built around Sahyadria and Dawkinsia sits in the cooler, well-oxygenated range, whereas Botia dario prefers warmer water and the Badidae need slow flow and dense cover. Combining species from different sub-regions or from incompatible temperature ranges departs from the biotope concept.

Rock, sand and leaf habitat

Seriously Fish describes the stream-style layout for the barbs as a substrate of variably sized, water-worn rocks, sand, fine gravel and some small boulders, with hardy plants such as Microsorum, Bolbitis and Anubias attached to the décor. For the Botia loaches it specifies sand or fine gravel, smooth water-worn rocks and pebbles, driftwood, plenty of cover, and sealed gaps so that fish cannot become trapped. The Badis badis layout adds floating plants, leaf litter and cave-like structures that double as spawning sites, and Dario dario needs dense stands of the stem plants recorded from its habitat. Botia dario and Botia almorhae jump, so a tight-fitting lid is essential.

Water movement and maintenance

For Sahyadria denisonii, Seriously Fish specifies a filter turnover of four to five times the tank volume per hour together with a high proportion of dissolved oxygen and moderate water movement; Dawkinsia filamentosa and Pethia conchonius benefit from a degree of movement without particularly strong filtration, and the Badidae favour low flow. Weekly water changes of 30–50% of the tank volume are given as routine for Sahyadria, Dawkinsia and Botia almorhae.

Simulating the seasons

A seasonal cycle can be represented by moving between the two phases the sources describe. The high-water phase corresponds to the monsoon months: larger and more frequent water changes, stronger circulation and the cooler end of each species' documented range (Badis badis and Sahyadria denisonii are both recorded down to 15 °C, and FishBase notes the cold tolerance of Pethia conchonius). The low-water phase corresponds to the dry season: reduced flow, water at the warmer end of the documented range, leaf litter and rock crevices as the shelter Botia use when congregating in pools. All changes must stay inside the ranges listed above, and the cool phase must not be applied to Botia dario, whose documented range starts at 23 °C.