*If the KGB gets the idea that there's something fishy about President Reagan's activities here this week, the KGB is dead right. Between briefings on his summit meeting with Soviet leader Mikhail Gorbachev, the leader of the free world has been spending a few minutes every day feeding goldfish.

Reagan took over the fish detail after he found a note awaiting him Saturday night at Maison de Saussure, where he and Nancy Reagan are staying.

The note was written by Hussain, the 11-year-old son of Prince Karim Aga Khan, and his wife, the Begum Salima, who lease the 18th-century mansion from a prominent Geneva physician, Daniel Pometta, and his wife.

Hussain keeps the goldfish in an aquarium in his second-floor bedroom and apparently became concerned about their future when he and his parents moved out to make way for the Reagans.

Elaine Crispen, Mrs. Reagan's press secretary, said the president, who is using the bedroom as a study, is taking Hussain's request very seriously and following the feeding schedule he left behind.

She did not say what the fish thought about the arrangement.

*Reagan is not exactly the magic name you might think. At Cointrin Airport Saturday night, Ron Reagan, the president's son, showed up for what had been billed as "open coverage" of his parents' arrival.

Reagan, covering the summit for Playboy magazine, quickly discovered that despite the advance billing, the Swiss had closed coverage of the event to all but a small group of representative reporters known as the pool. Without the necessary Swiss press credentials to flash at police, Ron Reagan was turned away just like any other reporter.

His status changed dramatically Sunday night when his parents had him over. Not for an exclusive interview, said Crispen. Just a little family dinner.

*Raisa Gorbachev's schedule on Wednesday will include a visit to a farm in nearby Celigny. It belongs to the Chenviere family and comes complete with cows, chickens, pigs, horses and other bucolic inhabitants.

The Soviet leader's wife reportedly had asked to see a "poor" farm, but they don't come that way in Switzerland, one of Europe's most prosperous countries.

In fact, even the Swiss joke that the difference between a poor farmer and a rich one is that the poor farmer washes his Mercedes at home and the rich one takes his to a car wash.

*On the fashion front: So far it's a draw. Nancy Reagan wore her full-length mink coat with matching hat when she and her husband were officially welcomed by Swiss President Kurt Furgler and his wife Ursula this afternoon, just before she had tea with Ursula Furgler and other Swiss officials' wives. Two hours later Raisa Gorbachev wore a gray wool coat with a large silver fox collar and matching fur hat when she and her husband were officially welcomed by the Furglers.

Under her coat, Nancy Reagan wore a red dress. It wasn't apparent what color dress Raisa Gorbachev was wearing. Perhaps red, white and blue?

*The Americans and Soviets are providing simultaneous translations in sessions opening Tuesday between Reagan and Gorbachev.

And that might be the end of it except, of course, that some wag wondered how Reagan, who wears a hearing aid, would be able to hear.

White House spokesman Larry Speakes didn't lose a beat. The White House was going to "implant" the microphone, quipped Speakes.

*Originally, Donna Hartman, wife of U.S. Ambassador to the U.S.S.R. Arthur Hartman, was on Nancy Reagan's guest list for Tuesday's tea with Raisa Gorbachev. Mrs. Reagan's office announced that Hartman would be the other American present.

Hartman flew here with her husband from Moscow, but today the White House said only Mrs. Reagan and Mrs. Gorbachev, with their interpreters, would be at the tea.

Spokesmen for Mrs. Reagan gave no explanation, other than that they had thought Mrs. Gorbachev might be accompanied by someone, perhaps Irina Dobrynin, the wife of Soviet Ambassador Anatoliy Dobrynin. But if Irina Dobrynin is in Geneva, no one on the American side has yet seen her.

*The price of Reagan-Gorbachev T-shirts has dropped even more than the prospects of peace, but even at that they're not cheap. On Sunday, the shirts were selling in the White House press filing center at the Hotel Intercontinental for $25 each. By today, the price had dropped to 25 Swiss francs, about $12.50.