3\. Chromis

Chromis are close relatives of damselfish and belong to the same family, Pomacentridae, but they are a world apart in temperament. Where damsels are territorial loners that stake out and defend a patch of rock, chromis are open-water schooling fish that spend their time hovering in loose groups in the middle of the water column, which brings a sense of constant, gentle movement to a reef tank that few other reef-safe fish can match. Because schooling behaviour is central to how they settle in, chromis should always be bought and kept in a group rather than as a single fish; a lone chromis, or a group that is too small for the tank it is in, tends to become stressed and can turn unexpectedly territorial, working against the very reason most people choose them.

Blue Green Chromis (Chromis viridis)

This is the chromis most commonly available in India, a small, iridescent fish that shifts between pale blue and green depending on the light. It is peaceful, reef safe, and among the easiest schooling marine fish to keep, doing well on a varied diet of small meaty frozen foods along with some algae-based food to round things out. A school should be introduced together, ideally five or six at a time, since adding chromis one at a time into an existing group tends to trigger squabbling as the pecking order gets renegotiated with each new arrival.

• Size: up to 8 to 10 cm
• Temperament: peaceful, schooling; mild squabbling can appear if the group is too small for the tank
• Minimum tank size: 150 litres for a school of five to six, more for larger groups
• Diet: omnivore, small meaty frozen foods plus occasional algae-based food
• Difficulty: easy, a very common and affordable choice for Indian reef tanks

Black Axil Chromis (Chromis atripectoralis)

Black Axil Chromis look extremely similar to the Blue Green Chromis at a glance, sharing the same pale blue to green iridescence, and the two are frequently mixed up in shops, sometimes even sold under the same generic label. The reliable identifying feature is a small dark mark at the base of the pectoral fin, the axil, which the Blue Green Chromis lacks, though this mark is not always visible on young fish and tends to darken and become obvious only as the fish matures, so a juvenile bought as one species can reveal its true identity a few months later. Its natural range covers the Indo-Pacific from the Red Sea and East African coast through to the central Pacific, which includes the wider Indian Ocean. Care requirements are much the same as its more common cousin, though Black Axil Chromis are reported to settle into somewhat smaller groups without the same degree of internal squabbling, while still tending to show more pronounced territorial behaviour once a hierarchy is established. As with any chromis, mixing species in the same school is best avoided, since a settled group can turn on an unfamiliar newcomer even of a closely related type.

• Size: up to 10 to 12 cm, slightly larger than Blue Green Chromis
• Temperament: peaceful to mildly territorial once a hierarchy forms
• Minimum tank size: 150 litres for a small group
• Diet: omnivore, same as Blue Green Chromis
• Difficulty: easy; check the pectoral fin base for the tell-tale dark mark to confirm identification before buying, keeping in mind it may not show clearly on smaller juveniles

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