Put some fish in a tank, cycle the water through a hydroponic grow bed and back to the fish tank, and you’ve got yourself a fish dinner with salad.

Two Hawaii Pacific University faculty members say it’s that easy to practice sustainable agriculture—and they’ve put together aquaponics gardens to show others how to do it. Aquaponics combines aquaculture (raising fish in tanks) and hydroponics (growing plants in a soilless medium). The fish tank water is pumped to a grow bed to fertilize the plants. In turn, the grow bed filters the water with its plant roots, algae and gravel, and the cleansed water cycles back into the fish tank.

"It’s an enclosed symbiotic relationship," said Lang ly Frissell, director of Distance Learning Programs at HPU, who also built an aquaponics garden in the back yard of his Kailua home.

"It’s incredibly gratifying to eat the food you’ve grown," Frissell said. "It tastes better."

Frissell, associate professor of microbiology Louis Primavera, their families and a handful of HPU students built a 15-by-80-foot demonstration garden at HPU’s Hawaii Loa campus last fall.

The garden was home to lettuce, herbs, bok choy, tomatoes, alfalfa, soybeans, watercress, cucumbers, strawberries and hundreds of tilapia. The vegetables were given to faculty members and their families. The tilapia can also be eaten, but their main purpose is to provide fertilizer for the plants, Primavera said.

"We’re hopeful that a family of four that eats vegetables will be able to pay off the cost of the aquaponics system in their back yard hopefully within six months to a year," he said.

"Supposedly you can grow as much produce in about 10 percent of the land," he said.

The goal of Primavera and Frissell is to see backyard aquaponics setups all over Oahu, or community aquaponics facilities for people without back yards. Frissell said aquaponics is scalable, so it can be used by one person or a community.

"This allows people not just to grow their own food, but to unite as communities in the process of building the apparatus," he said.

It’s being rebuilt this summer and the new system—which should be completed in about a month—will also have shrimp and prawns in the tank.

But because shrimps need salt water to survive, Primavera and his students are conducting experiments to see how salty the water can be and still support plant growth, and which plants grow best in brackish water.

"It’s my understanding that tomatoes can do that," he added.

Primavera said he would ultimately like to be able to provide food for the HPU cafeteria. He said the cafeteria manager is interested in using local produce, but doesn’t think the aquaponics system can keep up with their demand for lettuce and tomatoes.

"We’d like to accept that challenge," Frissell said.