The fish are very hungry. When Hendrik Monsees - a biologist at the Institute of Freshwater Ecology and Inland Fisheries in Berlin - throws dry food into the aquarium, the tilapia (a type of fish) immediately swim to the surface to snap it up. They fight for the food. Water is sprayed out of the chest-high aquarium. Monsees ducks for cover.

"When tilapia are hungry, there's a lot of spray," he says with a smile.

A few hundred tilapia are held in a dozen fish tanks at the institute. They are kept in a greenhouse where numerous tomato plants are grown. Air-conditioning keeps the temperature of the greenhouse constant at 27 degrees Celsius (81 degrees Fahrenheit) - making the environment conducive for tomatoes and tilapia. The aim of the project is to grow vegetables and farm fish in zero emission conditions.

Food production in a highly technical environment

Tomatoes and fish thrive in an artificial environment that uses technology to ensure optimal results.

The fish are kept humanely, says Werner Kloas who also happens to be the founder of the greenhouse. The tanks do not contain more fish than are normally found in their natural habitat - no more than a school of fish.

The water from the fish tanks feeds the tomatoes

And the tomatoes grow in an artificially enhanced environment. Instead of soil, they are planted in mineral wool.

"The cultivation of plants without soil is called hydroponics" Monsees explains. He moves a few leaves to one side and points to the small tomatoes. Growing tilapia and the vegetables has been successful as the researchers have demonstrated several times in the greenhouse.

This type of cultivation and farming is nothing new. Vegetables are grown in greenhouses around the world. And farming fish in tanks is nothing revolutionary. What is new is that water from the fish tanks is being used to grow the tomatoes. Aquaculture, the farming of aquatic organisms, and hydroponics - growing plants using mineral nutrient solutions in water, without soil - have been separate domains, Kloas explains.

Essentially, this project combines aquaculture and hydroponics.

"When the plants need water and the nutrients, it is delivered from the fish tanks," Kloas says.

Two step water purification

The fish excrete ammonia which is toxic for the Tilapia

Fish excrete ammonia, which is toxic for tilapia, so the water has to be treated. The advantage, however, is that the dirty water from the fish tank is an ideal fertilizer for tomatoes once the fish waste has been filtered and the ammonia has been chemically removed.

In the greenhouse, this takes place automatically. The dirty water from the fish tanks runs through white plastic pipes. During the first stage, fish waste is filtered out then the water is purified in a biofilter - a huge black tank with a small opening on the side.

Looking through an opening of the filter, Werner Kloas points to small plastic parts, floating around in the water. Despite being only about the size of the tip of the thumb, their surface area is greatly increased by the many holes that dot them. It is on this surface that a film of bacteria settles.

"That's bacteria – Nitrosomonas and nitrobacter – that is naturally found in air," explains Kloas. "One type is made from ammonium nitrite and the other from nitrite, ultimately nitrate."

And nitrate is an essential and valuable component of plant fertilizer.

The treated water from the fish tank is sent via pipes to the boxes where the tomato plants are growing. They extract nitrate from the water - any extra water is gotten rid of through the leaves as water vapor. Several cooling traps have been installed under the ceiling to ensure that the water condensation is directed back into the fish tanks. The fish-tomato cycle is now complete.

Saving water

The plants do not need fresh water unlike conventional fish farming and vegetable growing.

The tomatoes do not grow in soil, but mineral wool

"With such a closed loop system, we only need to use about 10-percent of fresh water a day," says Werner Kloas. "We can save so much water and therefore manage a sustainable system."

For drought-stricken areas in Africa, for example, the system would be a perfect and a profitable way to produce food. Evaporated water is extracted from the greenhouse via cooling traps, being re-fed back into the water system. However, the energy required for such a system is huge - at the institute, a photovoltaic plant has been installed to help limit emissions. Werner Kloas is sure that simple solar systems would be sufficient enough to provide energy all year around if such systems were installed in countries in Asia, Africa and South America. And experiments are already taking place in Berlin to see what other vegetables can be grown in such systems.