All sorts of people wander into the aquaponics hoop house at Cylburn Arboretum in Baltimore, where young scientists from the Johns Hopkins Center for a Livable Future are growing fish and plants.

There are people who dream of starting their own commercial operations. There are those who want to get off the grocery store grid and know the food they produce is pure and free of pesticides, antibiotics and genetic modifications. There are people who seek to put these systems in community gardens, at churches or in schools. And there are the curious biologists, botanists and physicists who want to see how the aquaponics lab put itself together.

It’s not clear how many of these 2,000 visitors have started their own operations, but at least a few are in the planning stages.

One recent visitor, Eric Hansen, had a schematic drawing of his system and had already spent $500 on PVC pipe connectors. Hansen, a construction engineer, took out his tape measure and ducked under the tanks, measuring pipes and examining netting. He put notes in his composition book next to his drawings.

“I definitely want to grow my own food, grow my own fish,” said Hansen, who already keeps a koi pond at his Owings Mills home. “The self-sustaining idea has been with me for 45, 50 years.”

Aquaponics, put simply, is a system that grows fish and plants. The fishes’ excrement fertilizes the plants, the plants clean the water, and the water recirculates so the cycle can begin again.

But simple things aren’t always simple: The Hopkins hoop house is a warren of pipes, barrels and floats that facilitate the biological processes. It’s not that you shouldn’t try this at home; it’s just that if you do, you might fail a few times before you figure out how to do it right.

The Hopkins lab has learned much in the last 18 months through trial and error. Like many other aquaculture startups, the scientists chose to grow tilapia because they’re fairly adaptable fish that will eat a variety of feed. They had planned to give their tilapia an all-plant diet, but then they were unable to get the quantities of vegetarian fish feed they needed at a price they could afford. They then realized that tilapias’ requirements for warm water was driving up their heating bills, especially after the winter Baltimore just endured. Blue gills might have been a better fish to try, said farm manager Laura Genello.

The plants have tested their patience as well. They tried tomatoes, but found the fruit split in the water. Better to stick with the hearty leafy greens.

Water from the four tilapia tanks passes through several clarifying tanks, which separate the solid from the liquids. The solids become fertilizer for a bed of marigolds sitting in gravel. The nitrogen in the liquid breaks down with the help of bacteria that rest on nets and becomes nitrate, which the plants can easily absorb. The system also pumps in dissolved oxygen. After a month or so, the tiny plants become verdant heads of kale, Swiss chard or romaine. Baltimoreans can buy the produce at the Waverly farmers’ market on Saturday mornings; it is part of Baltimore’s Farm City Alliance, a small inner-city network.

Genello said she sometimes has to warn entrepreneurs with stars in their eyes of the financial risks. Her farm-raised tilapia goes to the Hopkins dining hall, but those without the backing of a financial institution will have to compete with cheap imports from China, which has largely cornered the market.

“A lot of people see the appeal from the environmental perspective, I think, and want to make lots of money from this. But you really have to dial back those expectations,” Genello said.

There is also the matter of permits. It’s fairly simple to obtain one from the Maryland Department of Natural Resources to raise fish and sell them whole, Genello said. But if an aquaponics operation wanted to fillet them, there would be a different series of regulations from the Department of Health.

A better choice may be feeding the growing demand for leafy greens, or bartering with neighbors. The Hopkins romaine was crisp and fresh, and the kale stems burst with green.