The relaxing sound of trickling water echoes through Hank Brinzer’s attached greenhouse in a Pittsburgh suburb. Underneath a bench filled with lush plants, goldfish unknowingly feed seedlings as the water splashes into their tank.

It’s hydroponics with a fishy component.

“I’ve always been a tinkerer,” the semi-retired 67-year-old gardener says with a laugh.

Brinzer, who’s been gardening since age 14, wanted to try hydroponics and set up two types of growing systems in the greenhouse. Three months ago, on a visit a hydroponic-equipment store, he discovered aquaponics, in which fish water is used to feed the plants.

The system there is more complex than his, but the principles are the same.

His 30-gallon fish tank is equipped with a pump on a timer. Every three to four hours, water is pumped up into trays filled with plants. The water slowly drains through an overflow back into the fish tank below.

It’s a system known in the hydroponic community as “ebb and flow.” The plants live off what the fish provide; no other nutrients are added.

Judging from Brinzer’s deep-green plants and budding flowers, they are getting everything they need. As Brinzer slips a marigold plant out of its brown clay pot, he reveals thick white roots ready for the garden.

It’s a symbiotic relationship. The fish provide nutrients from their waste products, and the plants filter the water before it’s returned to the tank. Basically, bacteria break down the toxic ammonia in fish waste, turning it into nitrogen, one of the nutrients for growing plants.

The water is a little green, but the fish seem as happy as the plants they are helping to grow.

Through trial and error, Brinzer found what he needed to best support the plants. In his research, he found that the Aztecs used fish to feed their plants.

“It’s in its infancy again,” he says, “but it’s coming around.”

Brinzer spent only $30 for the trays and pump and about 49 cents each for the five or six goldfish. The fish tank came from a friend, and he already had the lighting.

Tomatoes, marigolds, herbs, peppers, cucumbers and cuttings of jasmine and geraniums are thriving in his aquaponics system. The only thing besides the cuttings not started from seed was a stevia plant.

Brinzer reflects on what it is that he gets out of gardening this way: “The joy of watching it grow, learning something new.”