When they started their farm near Pulau Ubin three years ago, Dr Michael Voigtmann and Dr Dirk Eichelberger knew that rearing fish in closed-containment systems was the way to go.

The Germans have pumped in $500,000 to invest in 40 such tanks and will be spending another $1 million to scale up their systems by end-June.

"We believe that net farming is simply too risky," said Dr Eichelberger, 51. "Besides the plankton bloom, you have other (aquatic) diseases. The water here is simply difficult to deal with."

Dr Voigtmann, 47, a Singapore permanent resident, said that apart from plankton blooms, fish also suffer from viral and bacterial diseases which may cause mortality. "You can vaccinate the fish, but it can't be protected from everything," he said. "Once we filter and treat the water with UV light, you can keep out almost all of the pathogens."

Their company - Singapore Aquaculture Technologies, was one of five awarded a tender by the Agri-Food and Veterinary Authority to design and develop closed-containment aquaculture systems.

Their system involves pumping seawater from a depth of 2m before filtering and passing it through UV light rays to kill almost all organisms, including plankton and bacteria.

For the record, the latest wave of plankton bloom on Feb 28 killed one-third of their fish stock, said Dr Eichelberger. But these were from tanks which did not have the UV systems.

"The good news for us, despite the disaster for so many farmers, is that the completed systems did their job and protected the fish."