You want to look at fish, go to the beach and jump in. Remember to hold your breath. But if fish-watching is your idea of relaxation and/or high-style decor, then you need an aquarium — the bigger the better.

If you’re interested in high-end aquariums, then mail-order is your only recourse. The choice is either the traditional glass tank from your local pet shop or a custom-built and installed aquarium.

"Some people have 200- to 300-gallon aquariums in their homes, but not many that I know," said Robert Lau of the Honolulu Aquarium Society. "You see the larger size mostly in hotels and restaurants."

The local experts we talked to say that when you’re approaching 300 gallons, you’re getting big.

And the larger the tanks are, the easier they are to maintain, according to Lau. That’s because, in smaller tanks, things can go south in a hurry if there are contaminants in the water, "such as from food or fish. The margin of error is greater."

"Anything can be built, any design you want," said Chris Otis, owner of Indoor Oceans, which designs and maintains aquariums in Hawaii. "The question is, will it work? Will there be the right amount of circulation and filtration? Will the water flow properly? If you spend $10,000 on having a custom tank built — and there are companies that will build you beautiful tanks, no questions asked — will it actually keep your fish alive? It’s always better to have a pro design it or advise on it."

Erick Umphress, owner of Reef Encounters, a company that designs fish tanks, agrees. "Anything can be built, any size you want, any shape. We’ve done it," said Umphress, a former marine engineer for Matson. "But it can’t be built in Hawaii. Everything these days is built of acrylic, or Plexiglas, and there’s too much moisture in Hawaiian air. Makes bubbles in the glue seams. Not good! So we build our tanks in the California desert, near Lancaster, and ship them over.

Fresh water or salt water?

"Most people go with salt, but in reality, in my opinion, the technology to keep the water clean is about the same for both these days," said Lau.

An aquarium is essentially a submarine turned inside out, a microcosm that exists in a delicate balance of natural materials.

For Otis, fresh water is "a no-brainer. Easy to maintain. But freshwater fish don’t have the color and personality of saltwater fish."

Saltwater aquariums often use "live rock" to help maintain the balance of chemicals. These are generally chunks of dead coral that are infested with microorganisms, and are often harvested from reefs in the Pacific.

"But it’s totally illegal in Hawaii (for commercial purposes)," said Lau. State statutes ban collecting, shipping, importing or selling live rock or coral in Hawaii. That makes faux coral and rock structures popular in large Hawaii saltwater tanks. And if you want some, you can collect some, but only for yourself and only if it’s already broken off reefs and the sea floor.

Here’s the relevant passage in state law:

"The taking of sand, coral rubble or other marine deposits is permitted in certain circumstances. The material may not exceed one gallon per person per day, and may be taken only for personal, noncommercial purposes."

So watch it. You’re on your own, rock-wise.

"In some ways, sometimes it’s easier just to change the water," said Umphress.

And a tank holding 300 gallons of water is also holding 2,505 pounds of water — more than a ton. How sturdy are your floors?

"In some ways, it’s easier to have the house built around the aquarium," said Otis. "If you want a big aquarium in a new house, the architect needs to know. Also, you can’t just put them anywhere. They can’t be in direct sunlight, for example."

A built-to-order big tank can easily cost between $5,000 and $10,000. Plus stands, hydraulics, purifiers, pumps and other ephemera. Plus water, fish and rocks.

Often, that’s just the beginning of the expense. Maintaining these tanks takes finesse on a regular basis. Otis says the primary income for his company comes from maintenance contracts.

"The folks who can afford such big tanks can generally afford to have someone else take care of them. Probably 90 percent of our installed tanks, we also maintain," he said.

A typical contract can cost up to $300 a month.