THE YEAR "1980 has been crazy," acknowledged Robert Moore, Washington's premier fishmonger. "It's crazy to open three places in a year."

Sitting down doesn't come easily to Moore, who owns and directs Cannon's Fish Market on 31st Street in Georgetown. But late last week he sat for a while and tried to explain why -- during a recession and in a time when the fish supply is threatened -- he has launched a trio of business enterprises: a retail fish store in Great Fall, Va.; a restaurant, Falls Landing, in the same shopping complex; and a stall in Georgetown's new Market House.

The connecting links are two: fish and the fact that Moore lives in Great Falls.

Fish, as we all know, has suddenly become something this nation of meateaters cannot do without. A specialty store once viable only in the city can now prosper in a suburban setting. Sadly, demand has grown at a time when fish is difficult to obtain and -- as proof that some classic economic theories still hold -- very expensive. Furthermore, we are very fussy about which fish we eat, which causes even more distortion in the supply-price equation and helps turn running a fish business from a problem into a headache. e

Moore lives with it, in part because he loves it and in part because he keeps up a schedule that would shame Washington's acclaimed white-collar workaholics. Since he left high school more than 20 years ago to work for his grandfather on Maine Avenue and then in the Western Market, 15- or 16-hour days have been the norm. He moved the business to Georgetown in 1966 and "slept on a desk here many a night" until it made a profit and he could pay off his debts. Since then he has slept at home but worked just as hard. In the process, he has developed a reputation for integrity and fairness. Cannon's is known in the fish supply business as a demanding customer.

In turn, Moore respects his customers. I try to go up and down the bench [retail display counter] in the middle of the day," Moore said, "to see what's out there.Sometimes there's a slip and we get fish from the middle of the catch [instead of the fresher top of the catch]. If it's not right, I'll hear about it. We have people in here who eat fish two or three times a week. When it's not right they really complain.

"We sell a lot more variety than we used to. I remember down on Maine Avenue [in the early 1960s] we had trouble giving stone crab claws away. These days people will try things they wouldn't then, but I have to be careful. Our customers, even the chefs, don't want the hassle of whole fish. Ninety percent of what we sell is fillets.

Despite the crowds at the counter, only about 25 percent of Cannon's business is retail. Most of the fish goes to embassies and restaurants and Moore observed with pleasure that "the competition among the top restaurants today seems to be over fish; not just fresh fish, but who has crayfish or soft shells and who doesn't. Those that don't lose out."

Moore has much the same selection of fish at his new Great Falls store plus an additional feature -- clambake packages containing fish and vegetables. pHe said he hasn't time to do them in Georgetown, where his wholesale operation is located. His involvement in the Great Falls complex where the store is located led him into the restaurant venture. The 150-seat facility opened in July. With its colonial decor and a first-rate chef, Falls Landing is too classy to be called a fish house. It offers specialties such as brochette of marinated swordfish, oysters Rockefeller and scallop soup in addition to the traditional lobster, crab and fin fish dishes.

Strangely enough, considering how much Moore knows as a major supplier of fish to restaurants, he did not base his menu at Great Falls on what sells elsewhere. "I went more on what our retail customers are buying," he said. "I want fast turnover. The fish is fresh except for shrimp, and I don't want it laying around in ice water for a few days before it's cooked. If seafood isn't buried in sauce, you can taste the difference when it's old."

The stall at Georgetown's Market House is to be something halfway between fish shop and restaurant. "We're going to have raw fish, but I've only got 500 square feet, so we won't have the selection I do at the other stores. The rest is going to be cooked food we'll make fresh every day, standup stuff like crab cakes and chowder and fish sandwiches that people can just buy and eat there or take home. Nothing with plates or silverware."

Moore made a promise to the Market House several years ago. When the project began to jell this spring, he honored his commitment in spite of his other burdens. Although the family has pitched in (his wife, Jackie, has been pressed into service at the Falls Landing restaurant and his teen-age children, Debbie and Bobby, both did counter duty in the Great Falls store this summer), it hasn't become any easier since then.

"Supply has been rough this summer," Moore said. "Last week we had only two or three kinds of whole fish available. That never happens." Then looking like a prime candidate to appear in an aspirin commercial, he ticked off several other difficulties.

Why do it all? he was asked. He shrugged. "How can you slow down when everything's going on around you?" he asked. The telephone rang. A second telephone rang.