''Did you know that fish don`t have eyelids?''

Insurance executive John Charles never would have considered the question a year ago, but now he`s hooked. He sits in one of the two armchairs that face his $10,000, 300-gallon aquarium. Up to his left is a gleaming glass bowl collection, which the fish were originally meant to complement.

''I liked the idea of having living jewels to go along with the glass jewels,'' Charles said. ''But it suddenly dawned on me that these were living creatures that you have to take care of. It didn`t take long to fall in love with the damn things.''

Charles is among the schools of recent converts to the charms of tropical sea creatures. Like some goofball product on ''Saturday Night Live'' (''It`s a floor wax-and a dessert topping!''), aquarium-residing fish have come to serve a multitude of purposes: They`re animated ornaments, stress reducers, status symbols, reminders of nature, microcosms for behavior studies, or simply little friends who open their mouths but can`t talk.

The American Veterinary Medical Association reports that as of 1988, 2.6 million households owned fish, an increase of 24 percent since 1983. Various studies put the nation`s total number of pet fish between 94.5 million and more than 250 million, making them the country`s most populous, if not most popular, pet.

(Although pet fish outnumber pet cats-current U.S. population 57.5 million-and pet dogs-population 49.9 million-the Pet Food Institute says that in 1988, 33.6 million households owned dogs and 27.3 million owned cats.)

''People are much more aware of how beautiful and exotic wildlife is, and they want it to be their own,'' said Linda Wilson, the Shedd Aquarium`s curator of education. Wilson and others involved in the fish industry said improved technology-better filters, increased knowledge of salt-water fish and feeding-and a trend toward conservation and re-creating natural habitats are further encouragements to bringing aquariums into homes.

Tropical fish ''are now seen as rare and valuable and something that needs to be seen and appreciated,'' Wilson said.

Their popularity is reflected in the Shedd Aquarium`s attendance, which broke 1 million in 1988. Aquarium spokeswoman Betsy Raymond said that one of the reasons for the aquarium`s new addition, the Oceanarium, which is scheduled to open in November, is that ''visitor surveys and Chicago-area surveys showed that people wanted to see more and learn more.''

Businesses such as Old Town Aquarium on North Wells Street also are netting the benefits. Old Town, which claims to be the highest-volume retailer of fish in the country and perhaps the world, sells a couple of thousand fish a week and expects to hit $2 million in sales this year, said Patty Oberg, who co-owns the store with her husband, Jim.

''Most fish hobbyists are fanatics-they`re eccentric enthusiasts,'' Oberg said, not excluding herself. But she added that many customers are people in pressure-cooker professions. ''All these professionals are seeking out aquariums as a way to escape the stress of their jobs.''

Special orders

Old Town does not specialize in guppies for yuppies, though it does sell them for less than $1 each. Oberg prides herself on providing fish that are caught with small nets-without using poisonous drugs to flush them out of hiding, which often results in unhealthy fish-from such spots as Fiji, Hawaii, the Indian Ocean, the Red Sea and the Great Barrier Reef.

''We have customers who fly here from Germany and Hong Kong,'' she said, adding that demand for special orders is rising. ''When I used to bring in Red Sea fish, it was mainly for show, like a museum. Now they`re gone the next day-and these are $600 to $800 fish.''

One regular high-end customer is Dean Linderman, a Dallas business executive who makes a point of visiting Old Town once every couple of months. Linderman estimated that after 15 years of collecting, he has 300 fish in 10 tanks that hold more than 1,400 gallons of salt water.

''I originally got in it because it was a hobby I could do at home when I was trying to juggle my responsibilities as a father to two kids and my career,'' he said. ''The real challenge in the hobby is getting more rare fish. You get pleasure knowing you have the only one of something in Texas or west of the Mississippi.''

Fish trader

A pet store frequenter, Linderman said he buys a new fish when he sees one he likes or gets a tip from Oberg, with whom he speaks almost weekly. ''If she calls with a fish I gotta have, I`ll pick the fish I like least and trade it in to one of the local pet stores,'' he said.

He declined to divulge how much he has spent on his hobby but noted: ''It is very easy between food and equipment and fish to spend $10,000 a year. In that will be a couple of $500 fish and lots of $200 fish.''

Although Linderman stressed that he is ''more than a casual hobbyist,''

he warned that any tropical-fish collector must be prepared to learn some difficult, costly lessons when starting out.

Since Charles, the insurance executive, had his tank installed in his North Side apartment last October, seven fish have died of disease, and he has witnessed a grisly example of why two similar specimens should not occupy the same tank.

Charles said he had two like-size, blue-and-orange fish of different species, and one chased the other ''till he was exhausted, and then they all ate him. I found his skeleton. That was the first fish I lost, and it was a hard one.''

Linderman noted, ''I have dropped fish in and had them get eaten in three seconds, but it`s only happened to me twice, and it wasn`t a consequential fish.''

Charles enjoys pointing out the similarities between his 10 fish that dart in and out of their coral, and people in a workplace. ''There`s always a pecking order,'' he said. For example, the clown trigger, looking like a character out of the animated film ''Yellow Submarine'' with its bright yellow lips and blotches covering its yellow-and-brown body, is the boss. ''Whenever I get a new fish in the tank,'' Charles said, ''he takes a nip out of its tail.''

Aquarium costs vary, depending on the scale. At Old Town, freshwater systems begin at $69 (10 gallons), but the most popular beginner`s saltwater tank (60 gallons) is $800, including filtration, air pump and other necessities. The saltwater payoff is that many of the most exotic and colorful specimens come from the sea.

Liz Sharp, co-manager of Aquariums By Design, on North Franklin Street, said some aquarium systems hit the million-dollar range. Her store has a bid pending to build a two-story aquarium that is ''going to be a whole wall, floor to ceiling.''