Q. I am interested in buying a fish that I saw at a local aquarium store — a clown sweetlips. It is brown with white spots and swims like a clownfish.

A friend of mine, who also has saltwater fish, tells me that he thinks they are difficult to keep, but the shop owner disagrees. I was interested in any information you could give me on its care.

A. The fish you are referring to is the clown or harlequin sweetlips, Plectorhinchus chaetodonoides. This species belongs to the family Haemulidae, which includes other sweetlips and grunts(so named for their ability to produce a grunting noise by grinding their pharyngeal teeth, which is amplified by the swimbladder).

There are nine species in the genus Plectorhinchus, several of which are commonly seen in aquarium stores. The most common is the clown sweetlips. It ranges from islands in the Indian Ocean, east to Samoa, north to southern Japan and south to Australia, and is typically found in waters 3 to 100 feet in depth. Juveniles — under about 3 inches — are brown overall with black-bordered white spots and white-and-black fins. As they grow they develop brown spots on a white background, which become more numerous as they age. Not only is there a dramatic color change, but there is also a radical transformation in size! Most aquarists do not realize that this species gets big, attaining a maximum length of 24 inches. Therefore, it is important that you have a sizable aquarium (135 gallons or larger) if you hope to raise this fish to adulthood.

Besides appearance, another endearing quality of this fish is its swimming behavior, which is similar to that of a clownfish. Juveniles undulate their bodies and scull with their enlarged pectoral fins when they swim or when they hang in the water column. These exaggerated swimming movements, as well as the juvenile coloration, apparently function to deter would-be predators.

It has been suggested that they look like a noxious, or distasteful, flatworm and are avoided by predators as a result. This type of mimicry — when a nontoxic species resembles a toxic one — is called Batesian mimicry.

Although mimicking a noxious flatworm may help prevent the sweetlips from becoming predator chow, it does not give it immunity from aggressive species in the aquarium. The big wagging tail and waving paired fins of this species are perfect targets for combative tankmates, such as damselfishes, hawkfishes, certain angelfishes, tiggerfishes, pufferfishes and porcupinefishes. Therefore, another prerequisite for maintaining a small clown sweetlips is to house it with passive species.

During the day these fish hide in crevices, under ledges, among branching corals and between or under coral boulders. At night they come out to feed — the adult's diet consisting of mollusks, crustaceans and small fish. Although no data exists on juvenile food habits, they probably feed on worms and small crustaceans.

Unfortunately, one of the biggest drawbacks to keeping this fish in captivity is getting it to feed. I have had the most success with live ghost or glass shrimp, often sold to freshwater aquarists. These shrimp are able to live in waters of a wide range of salinity and will survive in full-strength seawater. In order to induce the fish to feed it is best that your clown sweetlips is not kept with aggressive feeders that will also eat these shrimp, such as squirrelfishes, soldierfishes, groupers, snappers, goatfishes and large wrasses.

Once you induce feeding, food competitors can then be added. You can place the shrimp directly into the aquarium, but if it is packed with decor you may not be able to see if the fish locates and eats them. Try adding the shrimp in a shallow wide-mouthed jar, the opening of which should be big enough to allow the fish easy entry. Set the jar on the bottom of the aquarium with the mouth up. The shrimp will be unable to escape, but the fish will be able to swim in and eat them! Fortunately, as clown sweetlips mature they become less finicky about accepting aquarium fare.

Sometimes these fish will eat for weeks or months and then suddenly cease and die for no apparent reason. This may indicate the fish was captured with drugs (cyanide), the effects of which may not manifest themselves immediately. Death may also be the result of some key nutritional ingredient missing from their diets.

If you have to use glass or brine shrimp to get your sweetlips to feed, it is a good idea to wean them off live food and try and feed them a more varied diet. Soaking frozen or fresh foods in Selcon (a vitamin and fatty acid supplement from American Marine Inc.) before it is fed will also help ensure long-term health.

As mentioned above, juveniles are also somewhat reclusive during the day. Therefore, it is important to provide them with good hiding places to reduce stress levels.

In conclusion, I would have to agree with your friend — this is a sensitive fish that is not easy to maintain. Thus, I would recommend that you avoid this fish unless you are willing to meet the specific needs prescribed above.