An associate in a big Israeli advertising office has of late started telling people that he plans to fill his office aquarium with saltwater and to throw in some sharks as well. Although it is forbidden to import sharks into Israel, a saltwater aquarium is an achievable dream, even for someone with no previous experience in caring for fish.

Saltwater aquariums are meant to emulate a tropical ocean environment, characterized by high water temperatures. If the conditions are appropriate, such an aquarium can be a habitat for tropical fish of intense colors, which are considered more beautiful than freshwater fish.

Until more than a decade ago, attempts at caring for saltwater fish in private aquariums often met with failure. "Since then, we have learned a lot, the equipment has become more advanced and we have developed a good working method," says Shlomi Shalom from the Kfar Azar Atlantis aquarium and ornamental pools shop (www.atlantis-fish.com). "A saltwater aquarium presents a greater challenge, and getting it right provides you with a greater sense of fulfillment."

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Such aquariums need to be filled with coral found in tropical regions. According to Shalom this does not pose an ecological threat since most of the coral collected was strewn on the ocean bed. "These coral, for instance," he says while pointing at a certain aquarium, "came from a company that received a mining permit in the Caribbean. Some of the firm's profit goes to the rehabilitation of coral reefs."

The coral are not only used for ornamentation and as a shelter for the fish, they are also important for maintaining a high level of acidity in the water and act as biological filtering and cleaning agents. "The rock is porous and furrowed, increasing the surface that comes in contact with the water. In addition, the rock's furrows contain bacteria that break up organic substances and toxins," Shalom says.

It is possible to buy "live" coral or dry coral, which develop bacteria once they are submerged in water. Sandstone should be avoided since it contains minerals such as phosphate that easily dissolve in water and cause excessive algae formation.

Most buyers choose a saltwater aquarium called Marina. The more serious hobbyists advance to the Reef aquarium, which focuses on the corals. The temperature in the Reef aquarium has to be in the narrow range of 24 to 28 degrees Celsius - therefore it has to be heated in the winter and cooled in the summer. It also requires a pump that creates currents similar to ocean waves as well as powerful lighting (coral has a symbiotic relationship with algae, which require light for photosynthesis). This type of aquarium makes it more difficult to treat sick fish, since the medicine (antibiotics or copper-based drugs to counter parasites) can damage the coral. All these difficulties, combined with the expensive equipment and the limited variety of suitable fish, explain why most amateurs prefer the Marina aquarium.

One owner of a Marina variety aquarium paid a visit to Atlantis last Friday. "How are the fish - hanging on?" asks store owner Boogy (Yossi) Lavi. "Yes, but I have started to feel that my aquarium is inferior," answers the customer. "People get competitive," says Shlomi Shalom, "everyone wants their fish as big or as rare as possible."

When putting together an aquarium, you should first get detailed instructions from specialized stores. There are also Web forums for saltwater fish (for example, www.gocool.co.il and www.aqua.org.il). Equipment for the standard Marina aquarium can cost up to several thousand shekels. A 150-liter aquarium with lighting costs NIS 1,500-2,000, a current-creating pump NIS 150-500, a "live" sand bed NIS 200, 30 kilos of tropical rocks go for NIS 1,000-1,500 and a protein-breaker (for filtering) goes for NIS 500-2,000. You can fill the tank with sea water collected in uncontaminated areas. In addition you should get sea salt, a chlorine neutralizer, a salt level meter, a net and food - which, when added together, cost another several hundred shekels. The pros will then continue improving their equipment.

The aquarium's equipment has to be in use from two weeks to a month before any fish are put in. The cheapest fish (damsel) costs around NIS 18, and the more expensive ones may reach NIS 500. Yet most of the financial investment goes into the equipment - in part because a saltwater aquarium can contain only a few fish.

Ran Tsdaka from Petah Tikva has been a Marina aquarium owner for the past six months, after owning fresh water fish "on and off, since high school." Why switch to saltwater? "Honestly, my wife made me," he says. "Bottom line, they are very beautiful fish, fresh water fish are nowhere near as pretty." And what role does his wife play in the hobby? "I buy the fish and he handles the maintenance," says Shiri Tsdaka.

Tsdaka decided on a 200-liter aquarium on the grounds that "the bigger the volume, the less stress there is on the fish and the aquarium is easier to stabilize." At first, he says, "some fish did not adapt and some of them I put in too early. But now I have more experience and it all works out."

Which fish did they choose for the aquarium? "A tang [also known as a doctorfish or surgeonfish], a 9 [a blue fish with a black mark similar to the number 9], several clownfish, one damsel and a foxface - the biggest fish in the aquarium, roughly 15 cm. First thing in the morning I check the aquarium. When everything is in order it's the prettiest thing in the house. When I find a dead fish, it makes me very sad."